Reland '[msan] Intercept dladdr1, and refactor dladdr'

Relanding with #if SANITIZER_GLIBC to avoid breaking FreeBSD.
Also incorporates Arthur's BUILD.gn fix (thanks!) from https://reviews.llvm.org/rGc1e283851772ba494113311405d48cfb883751d1

Original commit message:
This patch adds an msan interceptor for dladdr1 (with support for RTLD_DL_LINKMAP and RTLD_DL_SYMENT) and an accompanying test. It also adds a helper file, msan_dl.cpp, that contains UnpoisonDllAddrInfo (refactored out of the dladdr interceptor) and UnpoisonDllAddr1ExtraInfo.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D154272