[gdb/testsuite] Fix gdb.reverse/i387-env-reverse.exp for -pie

[gdb/testsuite] Fix gdb.reverse/i387-env-reverse.exp for -pie

When running test-case gdb.reverse/i387-env-reverse.exp for x86_64-linux with
target board unix/-m32/-fPIE/-pie, we run into:
...
(gdb) PASS: gdb.reverse/i387-env-reverse.exp: push st0
info register eax^M
eax            0x56550000          1448411136^M
(gdb) FAIL: gdb.reverse/i387-env-reverse.exp: verify eax == 0x8040000
...

The problem is that the tested instruction (fstsw) only sets $ax, not $eax.

Fix this by verifying $ax instead of $eax.

Tested on x86_64-linux with target boards unix/-m32 and unix/-m32/-fPIE/-pie.