This commit adds architecture support for LoongArch.
The toolchain currently only supports the 64-bit target
loongarch64-unknown-linux-gnu.
It has been tested to build with GCC 12.1, GDB 12.1, Glibc 2.36, Linux
5.19 and Binutils 2.39 as of Aug 2022.
Signed-off-by: Jiajie Chen <c@jia.je>
