[ELF] Support mixed TLSDESC and TLS GD

We only support both TLSDESC and TLS GD for x86 so this is an x86-specific
problem. If both are used, only one R_X86_64_TLSDESC is produced and TLS GD
accesses will incorrectly reference R_X86_64_TLSDESC. Fix this by introducing
SymbolAux::tlsDescIdx.

Reviewed By: ikudrin

Differential Revision: https://reviews.llvm.org/D116900