[tests] Force (some) X86-specific tests to use an explicit triple

These tests are located in 'X86' subfolders which means that they should
be compiled for that target. As they did not have the target specified
explicitly, they in fact were compiled for a default target triple. Not
all targets support all required features for these tests; for example,
if NVPTX is used as a default triple, the tests fail. The patch makes the
tests run for 'x86_64', thus they pass regardless of the default target.

Differential Revision: https://reviews.llvm.org/D121998