This is a workaround to a problem in the 3.8 release that affects MIPS and
possibly other targets where the default is not supported but a sibling is
When TSAN_SUPPORTED_ARCH is not empty, cmake currently attempts to build a
tsan'd libcxx as well as test tsan for the default target regardless of whether
the default target is supported or not. This causes problems on MIPS32 since
tsan is supported for MIPS64 but not MIPS32.
There should be a better way to fix this but my attempts at a better solution
have run into difficulties (mostly related to preventing lit from testing the
default target). This at least solves the problem for now.