[ELF] Fix getRelocTargetVA formulae of R_TLS and R_NEG_TLS

For R_TLS:
1) Delete Sym.isTls() . The assembler ensures the symbol is STT_TLS.
   If not (the input is broken), we would crash (dereferencing null Out::TlsPhdr).
2) Change Sym.isUndefWeak() to Sym.isUndefined(), otherwise with --noinhibit-exec
   we would still evaluate the symbol and crash.
3) Return A if the symbol is undefined. This is PR40570.
   The case is probably unrealistic but returning A matches R_ABS and the
   behavior of several dynamic loaders.

R_NEG_TLS is obsoleted Sun TLS we don't fully support, but
R_RELAX_TLS_GD_TO_LE_NEG is still used by GD->LE relaxation (subl $var@tpoff,%eax).

They should add the addend. Unfortunately I can't test it as compilers don't seem to generate non-zero implicit addends.

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D62098

llvm-svn: 361146