[gdb/testsuite] Make gdb.arch/amd64-gs_base.exp unsupported for i386

[gdb/testsuite] Make gdb.arch/amd64-gs_base.exp unsupported for i386

With target board unix/-m32 I run into:
...
(gdb) print /x $fs_base^M
$1 = void^M
(gdb) FAIL: gdb.arch/amd64-gs_base.exp: print fs_base
...

The problem is that the fs_base register is not supported for i386.

Fix this by making the test unsupported if fs_base/gs_base don't show up in
info register sys output.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-12-04  Tom de Vries  <tdevries@suse.de>

	PR testsuite/26990
	* gdb.arch/amd64-gs_base.exp: Handle -m32 where fs_base and gs_base
	are unsupported.