[clang-cl] Fix PCH tests to use x86_64 as target

These tests require x86-registered-target, but they don't force the target as
x86 on the command line, which means they will be run and they might fail when
building the x86 backend on another platform (such as AArch64).

Fixes https://llvm.org/bugs/show_bug.cgi?id=28797

Differential Revision: https://reviews.llvm.org/D23054

llvm-svn: 277457