[llvm-nm][test] Move X86 lit.local.cfg into the X86 subfolder

The file seems to have been put in the wrong place in its original
commit. This had the effect of marking all llvm-nm tests as unsupported,
unless X86 was enabled, even for tests that weren't X86 specific.

Fixes https://bugs.llvm.org/show_bug.cgi?id=52506.

Reviewed by: mstorsjo

Differential Revision: https://reviews.llvm.org/D113882