[ORC][AArch64] Add initial support for aarch64 in ELFNixPlatform.

Adds the aarch64 support in ELFNixPlatform. These are few simple changes, but it allows us to use the orc runtime in ELF/AARCH64 backend. It succesfully run the static initializers of stdlibc++ iostream so that "cout << Hello world" testcase starts to work.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D127060