[yaml2obj] Support PT_GNU_STACK and PT_GNU_RELRO

PT_GNU_STACK is used in an llvm-objcopy test.

I plan to use PT_GNU_RELRO in a patch to improve nested segment
processing in llvm-objcopy (PR42963).

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D67146

llvm-svn: 370857