[ARM][AArch64] Pass through endian flags to assembler and linker.

The big-endian arm32 Linux builds are currently failing when the
-mbig-endian flag is used but the binutils default on the system is little
endian. This also holds when -mlittle-endian is used and the binutils
default is big endian.

The patch always passes through -EL or -BE to the assembler and linker,
taking into account the target and the -mbig-endian and -mlittle-endian
flag.

Fixes pr38770

Differential Revision: https://reviews.llvm.org/D52784

llvm-svn: 344597