[compiler-rt] Fix broken sanitizer bots (hopefully)
According to the logs and local debugging there were two issues:
- tsan tests listed libc++.a before the source file. That's usually ok for shared libraries, but the linker will not add symbols from a static library unless needed at that time. As a result the tests that rely upon symbols from the library (and not only include the headers) had undefined references. To solve this I'm adding a new substitution %link_libcxx_tsan which expands to libc++.a if available.
- The target Fuzzer-x86_64-Test linked in SANITIZER_TEST_CXX_LIBRARIES which defaults to -lstdc++. This resulted in error messages like hidden symbol '_ZdlPv' is not defined locally hidden symbol '_Znwm' is not defined locally when using GNU gold (ld.bfd and lld are fine). Removing the linkage is fine because we build a custom libc++ for that purpose.