test/Driver/fsanitize.c: Fix -fsanitize=vptr using default target
In revision rL365872, a line containing '-target x86_64-linux-gnu' along
with '-fsanitize=vptr' was modified to only contain the -fsanitize option.
The default implementation of getSupportedSanitizers isn't able to turn
on the vptr sanitizer, and thus, any platform that runs this test will
fail with the error:
clang: error: unsupported option '-fsanitize=vptr' for target '<target>'