gdb/testsuite: Disable gdb.compile when testing with clang

Attempting to test the gdb.compile with clang as the compiler results in
over 300 unexpected errors, due to a segmentation fault and several
handshake failures. Since the whole feature is designed around a gcc
plugin, and even the gcc testing is shaky at best, this commit restricts
those tests to only running under gcc. If that gets fixed, this commit
can be reverted.

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