[lld] Add REQUIRES: x86 where needed to tests

If building lld without x86 support, tests that require that support should
be treated as unsupported, not errors.

Tested using:
  1. cmake '-DLLVM_TARGETS_TO_BUILD=AArch64;X86'
     make check-lld
     =>
     Expected Passes    : 1406
     Unsupported Tests  : 287

  2. cmake '-DLLVM_TARGETS_TO_BUILD=AArch64'
     make check-lld
     =>
     Expected Passes    : 410
     Unsupported Tests  : 1283

Patch by Joel Jones

Differential Revision: https://reviews.llvm.org/D47748

llvm-svn: 334095