[asan] Stop instrumenting user-defined ELF sections

Do not instrument user-defined ELF sections (whose names resemble valid
C identifiers).  They may have special use semantics and modifying them
may break programs.  This is e.g. the case with NetBSD __link_set API
that expects these sections to store consecutive array elements.

Differential Revision: https://reviews.llvm.org/D76665