[ELF] Implements -z relro: create an ELF "PT_GNU_RELRO" segment header in the object.

Partial (-z relro) and full (-z relro, -z now) relro cases are implemented.

Partial relro:
The ELF sections are reordered so that the ELF internal data sections (.got, .dtors, etc.) precede the program's data sections (.data and .bss).
.got is readonly, .got.plt is still writeable.

Full relro:
Supports all the features of partial RELRO, .got.plt is also readonly.

Differential revision: http://reviews.llvm.org/D14218

llvm-svn: 253967