[PPC64] Thread-local storage general-dynamic to initial-exec relaxation.

Patch adds support for relaxing the general-dynamic tls sequence to
initial-exec.

the relaxation performs the following transformation:
addis r3, r2, x@got@tlsgd@ha --> addis r3, r2, x@got@tprel@ha
addi r3, r3, x@got@tlsgd@l   --> ld r3, x@got@tprel@l(r3)
bl __tls_get_addr(x@tlsgd)   --> nop
nop                          --> add r3, r3, r13

and instead of emitting a DTPMOD64/DTPREL64 pair for x, we emit a single
R_PPC64_TPREL64.

Differential Revision: https://reviews.llvm.org/D48090

llvm-svn: 335651