[gdb/testsuite] Require have_compile_flag -mavx512f in gdb.arch/i386-avx512.exp

When running test-case gdb.arch/i386-avx512.exp with gcc 4.8.4, I run into:
...
Running gdb.arch/i386-avx512.exp ...
gdb compile failed, gcc: error: unrecognized command line option '-mavx512f'
...

Fix this by requiring have_compile_flag -mavx512f.

Tested on x86_64-linux.