[gdb/testsuite] Fix gdb.arch/i386-signal.exp on x86_64

On x86_64-linux, with test-case gdb.arch/i386-signal.exp I run into:
...
builtin_spawn -ignore SIGHUP gcc -fno-stack-protector i386-signal.c \
  -fdiagnostics-color=never -fno-pie -g -no-pie -lm -o i386-signal^M
/tmp/cc2xydTG.s: Assembler messages:^M
/tmp/cc2xydTG.s:50: Error: operand size mismatch for `push'^M
compiler exited with status 1
output is:
/tmp/cc2xydTG.s: Assembler messages:^M
/tmp/cc2xydTG.s:50: Error: operand size mismatch for `push'^M

gdb compile failed, /tmp/cc2xydTG.s: Assembler messages:
/tmp/cc2xydTG.s:50: Error: operand size mismatch for `push'
UNTESTED: gdb.arch/i386-signal.exp: failed to compile
...

This is with gas 2.41, it compiles without problems with gas 2.40.  Some more
strict checking was added in commit 5cc00775 ("x86: further adjust
extend-to-32bit-address conditions").  This may or may not be a gas regression
( https://sourceware.org/pipermail/binutils/2023-October/129818.html ).

The offending bit is:
...
    "    push $sigframe\n"
...
which refers to a function:
...
    "    .globl sigframe\n"
    "sigframe:\n"
...

The test-case passes with target board unix/-m32.

Make the test-case work by using pushq instead of push for the
is_amd64_regs_target case.

Tested on x86_64-linux, with target boards:
- unix/-m64 (is_amd64_regs_target == 1), and
- unix/-m32 (is_amd64_regs_target == 0),

PR testsuite/30928
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30928