[ELF] Resolve weak undefined TLS symbols when no phdr is available.

If we pass --gc-sections to lld and .tbss is not referenced,
the section is reclaimed and lld doesn't create a TLS program header.
R_TLS tries to access the program header -> lld crashes.
Mimic what bfd/gold do in this case and resolve a weak undefined
TLS symbol to the base of the TLS block, i.e. give it a value of zero.

Differential Revision:  https://reviews.llvm.org/D24832

llvm-svn: 282279