[gdb/testsuite] Fix g0 search in gdb.arch/i386-sse-stack-align.exp

When running test-case gdb.arch/i386-sse-stack-align.exp on target board
unix/-m32, I run into:
...
(gdb) print (int) g0 ()^M
Invalid data type for function to be called.^M
(gdb) FAIL: gdb.arch/i386-sse-stack-align.exp: print (int) g0 ()
...

Gdb is supposed to use minimal symbol g0:
...
$ nm i386-sse-stack-align | grep g0
08049194 t g0
...
but instead it finds a g0 symbol in the debug info of libm, specifically in
./sysdeps/ieee754/ldbl-96/e_lgammal_r.c.

Fix this by renaming g[0-4] to test_g[0-4].

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

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.