Add support for LoongArch
I don't know if GitLab MRs are the preferred way to submit changes now, so apologizes ahead and I'll switch to the mailing list if that's still the right way to contribute.
This MR adds support for building under the LoongArch architecture; information about this architecture can be found in the links below.
- https://github.com/loongson/LoongArch-Documentation - official translated documentation
- https://github.com/sunhaiyong1978/CLFS-for-LoongArch - tutorial and pre-built image suitable for LoongArch (Chinese-only)
- https://github.com/loongson/binutils-gdb/tree/loongarch/upstream_v6_a1d65b3 - latest binutils upstream submission
- https://github.com/loongson/gcc/tree/loongarch_upstream - latest gcc port for upstream
The EM_LOONGARCH constant is already approved earlier, and the built-in #defined names are expected to be supported both by their early in-house version and upstream versions. 32-bit support is incomplete and redacted by Loongson, so this MR probes for 64-bit only.