[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.