[gdb/testsuite] Fix gdb.arch/i386-gnu-cfi.exp

When running test-case gdb.arch/i386-gnu-cfi.exp with target board unix/-m32, I get:
...
(gdb) up 3^M
79      abort.c: No such file or directory.^M
(gdb) FAIL: gdb.arch/i386-gnu-cfi.exp: shift up to the modified frame
...

The preceding backtrace looks like this:
...
(gdb) bt^M
 #0  0xf7fcf549 in __kernel_vsyscall ()^M
 #1  0xf7ce8896 in __libc_signal_restore_set (set=0xffffc3bc) at \
     ../sysdeps/unix/sysv/linux/internal-signals.h:104^M
 #2  __GI_raise (sig=6) at ../sysdeps/unix/sysv/linux/raise.c:47^M
 #3  0xf7cd0314 in __GI_abort () at abort.c:79^M
 #4  0x0804919f in gate (gate=0x8049040 <abort@plt>, data=0x0) at gate.c:3^M
 #5  0x08049176 in main () at i386-gnu-cfi.c:27^M
...
with function gate at position #4, while on another system where the test passes,
I see instead function gate at position #3.

Fix this by capturing the position of function gate in the backtrace, and
using that in the rest of the test instead of hardcoded constant 3.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-01-28  Tom de Vries  <tdevries@suse.de>

	* gdb.arch/i386-gnu-cfi.exp: Capture the position of function gate
	in the backtrace, and use that in the rest of the test instead of
	hardcoded constant 3.  Use "frame" instead of "up" for robustness.

2 files changed, 25 insertions, 7 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index ac45381..73bbd00 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,11 @@
 2021-01-28  Tom de Vries  <tdevries@suse.de>
 
+	* gdb.arch/i386-gnu-cfi.exp: Capture the position of function gate
+	in the backtrace, and use that in the rest of the test instead of
+	hardcoded constant 3.  Use "frame" instead of "up" for robustness.
+
+2021-01-28  Tom de Vries  <tdevries@suse.de>
+
 	* gdb.arch/i386-sse-stack-align.S: Rename g[0-4] to test_g[0-4].
 	* gdb.arch/i386-sse-stack-align.c: Same.
 	* gdb.arch/i386-sse-stack-align.exp: Same.
diff --git a/gdb/testsuite/gdb.arch/i386-gnu-cfi.exp b/gdb/testsuite/gdb.arch/i386-gnu-cfi.exp
index 4228140..657599b 100644
--- a/gdb/testsuite/gdb.arch/i386-gnu-cfi.exp
+++ b/gdb/testsuite/gdb.arch/i386-gnu-cfi.exp
@@ -58,21 +58,33 @@ gdb_test_multiple {} "continue to abort()" {
     }
 }
 
-# Backtrace would crash before.
-
-gdb_test "bt" \
-         "main \\(\\) at .*${testfile}.c.*" \
-         "backtrace"
+# Verify that the backtrace works (it would crash before).  Use the backtrace
+# output to find the frame number for function "gate".
+set gate_n ""
+set backtrace_re \
+    [multi_line \
+	 "" \
+	 "#($decimal) *$hex in gate \[^\r\n\]*" \
+	 "#$decimal *$hex in main \\(\\) at .*${testfile}.c.*"]
+gdb_test_multiple "backtrace" "" {
+    -re -wrap $backtrace_re {
+	set gate_n $expect_out(1,string)
+	pass $gdb_test_name
+    }
+}
+if { $gate_n == "" } {
+    return -1
+}
 
 # Check we see the inserted `DW_CFA_GNU_negative_offset_extended' CFI.
 # We see there the original CFI-stored filename `gate.c'.
 
-gdb_test "up 3" \
+gdb_test "frame $gate_n" \
          "gate \\(\[^()\]*\\) at .*gate.c.*" \
          "shift up to the modified frame"
 
 gdb_test_multiple "info frame" "existence of the CFI inserted register" {
-    -re "Stack level 3, frame at (0x\[0-9a-f\]+):.*Saved registers:.* ecx at (0x\[0-9a-f\]+),.*" {
+    -re "Stack level $gate_n, frame at (0x\[0-9a-f\]+):.*Saved registers:.* ecx at (0x\[0-9a-f\]+),.*" {
 	pass "existence of the CFI inserted register"
 	if { [string compare $expect_out(1,string) $expect_out(2,string)] } then {
 	    fail "value of the CFI inserted register"