[Support/ELF] - Added DT_TLSDESC_PLT and DT_TLSDESC_GOT constants.
Added 2 constants:
DT_TLSDESC_PLT = 0x6FFFFEF6, Location of PLT entry for TLS descriptor resolver calls.
DT_TLSDESC_GOT = 0x6FFFFEF7, Location of GOT entry used by TLS descriptor resolver PLT entry.
Constants were taken from "Thread-Local Storage Descriptors for IA32 and AMD64/EM64T Version 0.9.5" http://www.fsfla.org/~lxoliva/writeups/TLS/RFC-TLSDESC-x86.txt
Differential revision: http://reviews.llvm.org/D16185