[compiler-rt] Fix broken sanitizer bots (hopefully)

According to the logs and local debugging there were two issues:
1) 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.
2) 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.

llvm-svn: 354231

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