loongarch64: new target

loongarch64: new target

Add target for Loongson LoongArch64-based boards.

LoongArch is a new RISC ISA developed by Loongson. It's a bit like
MIPS or RISC-V. LoongArch includes both 32-bit and 64-bit versions
(LoongArch32/LoongArch64).

Loongson 3A5000 and 3A6000 are the two existing CPUs of LoongArch64
and is used for PC products. It's BIOS supports ACPI and UEFI-only
boot. These CPUs supports SMP and SMT.

At present only LoongArch64 is supported by linux kernel.

Toolchain requirement:
binutils >= 2.40
gcc >= 13.1

For details, please check the following links:
https://lwn.net/Articles/861951/
https://loongson.github.io/LoongArch-Documentation/README-EN.html

Signed-off-by: Weijie Gao <hackpascal@gmail.com>