elf: Use official name LoongArch for EM_LOONGARCH.

elf: Use official name LoongArch for EM_LOONGARCH.

The official name for Loongson Architecture is LoongArch, it is better
to use LoongArch instead of Loongson Loongarch for EM_LOONGARCH to avoid
confusion and keep consistent with the various of software in the future.

The official documentation in Chinese:
http://www.loongson.cn/uploadfile/cpu/LoongArch.pdf

The translated version in English:
https://loongson.github.io/LoongArch-Documentation/

    binutils/
        * readelf.c (get_machine_name): Change Loongson Loongarch to
	LoongArch.

    include/
        * elf/common.h (EM_LOONGARCH): Change Loongson Loongarch to
	LoongArch.