gcc/configure: Fix check for assembler section merging support on Arm

In 32-bit Arm assembly, the @ character is the start of a comment so
the section type needs to use the % character instead.

configure.ac attempts to account for this difference by doing a second
try when checking the assembler for section merging support.
Unfortunately there is a bug: because the gcc_GAS_CHECK_FEATURE macro
has a call to AC_CACHE_CHECK, it will actually skip the second try
because the gcc_cv_as_shf_merge variable has already been set:

  checking assembler for section merging support... no
  checking assembler for section merging support... (cached) no

Fix by using a separate variable for the second try, as is done in the
check for COMDAT group support.

This problem was noticed because the recent binutils commit
d5cbf916be4a ("gas/ELF: also reject merge entity size being zero") caused
gas to be stricter about mergeable sections without an entity size:

configure:27013: checking assembler for section merging support
configure:27022: /path/to/as   --fatal-warnings -o conftest.o conftest.s >&5
conftest.s: Assembler messages:
conftest.s:1: Warning: invalid merge / string entity size
conftest.s: Error: 1 warning, treating warnings as errors
configure:27025: $? = 1
configure: failed program was
.section .rodata.str, "aMS", @progbits, 1
configure:27036: result: no

In previous versions of gas the conftest.s program above was accepted
and configure detected support for section merging.

See also:
https://linaro.atlassian.net/browse/GNU-1427
https://sourceware.org/bugzilla/show_bug.cgi?id=32491

Tested on armv8l-linux-gnueabihf.

gcc/ChangeLog:
	* configure.ac: Fix check for HAVE_GAS_SHF_MERGE on Arm targets.
	* configure: Regenerate.

2 files changed, 13 insertions, 10 deletions

diff --git a/gcc/configure b/gcc/configure
index a495762..b4c52de 100755
--- a/gcc/configure
+++ b/gcc/configure
@@ -27038,12 +27038,12 @@ $as_echo "$gcc_cv_as_shf_merge" >&6; }
 
 
 if test $gcc_cv_as_shf_merge = no; then
-  { $as_echo "$as_me:${as_lineno-$LINENO}: checking assembler for section merging support" >&5
-$as_echo_n "checking assembler for section merging support... " >&6; }
-if ${gcc_cv_as_shf_merge+:} false; then :
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking assembler for section merging support (%progbits)" >&5
+$as_echo_n "checking assembler for section merging support (%progbits)... " >&6; }
+if ${gcc_cv_as_shf_merge_percent+:} false; then :
   $as_echo_n "(cached) " >&6
 else
-  gcc_cv_as_shf_merge=no
+  gcc_cv_as_shf_merge_percent=no
   if test x$gcc_cv_as != x; then
     $as_echo '.section .rodata.str, "aMS", %progbits, 1' > conftest.s
     if { ac_try='$gcc_cv_as $gcc_cv_as_flags --fatal-warnings -o conftest.o conftest.s >&5'
@@ -27053,7 +27053,7 @@ else
   $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
   test $ac_status = 0; }; }
     then
-	gcc_cv_as_shf_merge=yes
+	gcc_cv_as_shf_merge_percent=yes
     else
       echo "configure: failed program was" >&5
       cat conftest.s >&5
@@ -27061,14 +27061,15 @@ else
     rm -f conftest.o conftest.s
   fi
 fi
-{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $gcc_cv_as_shf_merge" >&5
-$as_echo "$gcc_cv_as_shf_merge" >&6; }
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $gcc_cv_as_shf_merge_percent" >&5
+$as_echo "$gcc_cv_as_shf_merge_percent" >&6; }
 
 
 fi
 
 cat >>confdefs.h <<_ACEOF
-#define HAVE_GAS_SHF_MERGE `if test $gcc_cv_as_shf_merge = yes; then echo 1; else echo 0; fi`
+#define HAVE_GAS_SHF_MERGE `if test $gcc_cv_as_shf_merge = yes \
+    || test $gcc_cv_as_shf_merge_percent = yes; then echo 1; else echo 0; fi`
 _ACEOF
 
 
diff --git a/gcc/configure.ac b/gcc/configure.ac
index 2426163..6c38c49 100644
--- a/gcc/configure.ac
+++ b/gcc/configure.ac
@@ -3612,12 +3612,14 @@ gcc_GAS_CHECK_FEATURE(section merging support, gcc_cv_as_shf_merge,
  [--fatal-warnings],
  [.section .rodata.str, "aMS", @progbits, 1])
 if test $gcc_cv_as_shf_merge = no; then
-  gcc_GAS_CHECK_FEATURE(section merging support, gcc_cv_as_shf_merge,
+  gcc_GAS_CHECK_FEATURE(section merging support (%progbits),
+    gcc_cv_as_shf_merge_percent,
     [--fatal-warnings],
     [.section .rodata.str, "aMS", %progbits, 1])
 fi
 AC_DEFINE_UNQUOTED(HAVE_GAS_SHF_MERGE,
-  [`if test $gcc_cv_as_shf_merge = yes; then echo 1; else echo 0; fi`],
+  [`if test $gcc_cv_as_shf_merge = yes \
+    || test $gcc_cv_as_shf_merge_percent = yes; then echo 1; else echo 0; fi`],
 [Define 0/1 if your assembler supports marking sections with SHF_MERGE flag.])
 
 gcc_GAS_CHECK_FEATURE([COMDAT group support (GNU as)],