[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:
- cmake '-DLLVM_TARGETS_TO_BUILD=AArch64;X86' make check-lld => Expected Passes : 1406 Unsupported Tests : 287
- 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