[msan] Hotfix clang/test/CodeGen/sanitize-memory-disable.c

Because KMSAN is not supported on many architectures, explicitly build
the test with -target x86_64-linux-gnu.

Fixes the 'unsupported architecture' and 'unsupported operating system'
errors reported by the clang-armv7-quick (https://lab.llvm.org/buildbot#builders/171/builds/2595)
and llvm-clang-x86_64-sie-ubuntu-fast (https://lab.llvm.org/buildbot#builders/139/builds/9079)
builders.

Differential Revision: https://reviews.llvm.org/D108465