[clang/test] Run thinlto-clang-diagnostic-handler-in-be.c on x86

Clang test CodeGen/thinlto-clang-diagnostic-handler-in-be.c fails on
some non x86 targets, e.g. hexagon. Since the test already requires x86
to be available as a target this commit forces the target to x86_64.

Reviewed By: steven_wu

Differential Revision: https://reviews.llvm.org/D107667