gdb.arch/amd64-gs_base.exp: Support non-Linux.

The orig_rax pseudo-register is Linux-specific and isn't relevant to
this test.  The fs_base and gs_base registers are also not treated as
system registers in other OS ABIs.  This allows the test to pass on
FreeBSD.

Reviewed-By: Tom Tromey <tom@tromey.com>