libatomic/test: Fix compilation for build sysroot

Fix a problem with the libatomic testsuite using a method to determine
the compiler to use resulting in the tool being different from one the
library has been built with, and causing a catastrophic failure from the
lack of a suitable `--sysroot=' option where the `--with-build-sysroot='
configuration option has been used to build the compiler resulting in
the inability to link executables.

Address this problem by providing a DejaGNU configuration file defining
the compiler to use, via the GCC_UNDER_TEST TCL variable, set from $CC
by autoconf, which will have all the required options set for the target
compiler to build executables in the environment configured, removing
failures like:

.../bin/riscv64-linux-gnu-ld: cannot find crt1.o: No such file or directory
.../bin/riscv64-linux-gnu-ld: cannot find -lm
collect2: error: ld returned 1 exit status
compiler exited with status 1
FAIL: libatomic.c/atomic-compare-exchange-1.c (test for excess errors)
Excess errors:
.../bin/riscv64-linux-gnu-ld: cannot find crt1.o: No such file or directory
.../bin/riscv64-linux-gnu-ld: cannot find -lm

UNRESOLVED: libatomic.c/atomic-compare-exchange-1.c compilation failed to produce executable

and bringing overall test results for the `riscv64-linux-gnu' target
(here with the `x86_64-linux-gnu' host and RISC-V QEMU in the Linux user
emulation mode as the target board) from:

		=== libatomic Summary ===

# of unexpected failures	27
# of unresolved testcases	27

to:

		=== libatomic Summary ===

# of expected passes		54

	libatomic/
	* configure.ac: Add testsuite/libatomic-site-extra.exp to output
	files.
	* configure: Regenerate.
	* libatomic/testsuite/libatomic-site-extra.exp.in: New file.
	* testsuite/Makefile.am (EXTRA_DEJAGNU_SITE_CONFIG): New
	variable.
	* testsuite/Makefile.in: Regenerate.

1 files changed, 3 insertions, 0 deletions

diff --git a/libatomic/configure b/libatomic/configure
index 8cad97e..3e2f2ff4 100755
--- a/libatomic/configure
+++ b/libatomic/configure
@@ -15728,6 +15728,8 @@ fi
 
 ac_config_files="$ac_config_files Makefile testsuite/Makefile"
 
+ac_config_files="$ac_config_files testsuite/libatomic-site-extra.exp"
+
 cat >confcache <<\_ACEOF
 # This file is a shell script that caches the results of configure
 # tests run on this system so they can be shared between configure
@@ -16799,6 +16801,7 @@ do
     "gstdint.h") CONFIG_COMMANDS="$CONFIG_COMMANDS gstdint.h" ;;
     "Makefile") CONFIG_FILES="$CONFIG_FILES Makefile" ;;
     "testsuite/Makefile") CONFIG_FILES="$CONFIG_FILES testsuite/Makefile" ;;
+    "testsuite/libatomic-site-extra.exp") CONFIG_FILES="$CONFIG_FILES testsuite/libatomic-site-extra.exp" ;;
 
   *) as_fn_error $? "invalid argument: \`$ac_config_target'" "$LINENO" 5;;
   esac