[gdb/testsuite] Enable some test-cases for x86_64 -m32

[gdb/testsuite] Enable some test-cases for x86_64 -m32

When trying to run test-case gdb.reverse/i387-env-reverse.exp for x86_64-linux
with target board unix/-m32, it's skipped.

Fix this by using is_x86_like_target instead of istarget "i?86-*linux*".

This exposes a number of duplicates, fix those by making the test names unique.

Likewise in a couple of other test-cases.

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