[ELF] - Disable relro when -omagic specified.

--omagic is an option to create old-fashioned executables in which
.text segments are writable. Today, the option is still in use to
create special-purpose programs such as boot loaders. It doesn't
make sense to create PT_GNU_RELRO for such executables.

DIfferential revision: https://reviews.llvm.org/D27297

llvm-svn: 288579