[Aarch64] Fix linker emulation for Aarch64 big endian

This patch fixes target linker emulation for aarch64 big endian.
aarch64_be_linux is not recognized by gnu ld. The equivalent emulation
mode supported by gnu ld is aarch64linuxb.

Patch by: Bharathi Seshadri

Reviewed by: Peter Smith

Differential Revision: https://reviews.llvm.org/D42930

llvm-svn: 341312