elf: Add machine type value for LoongArch

This is already officially allocated as recorded in GNU binutils
repo [1], and the description is updated in [2]. Add to enable further
work.

[1]: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=4cf2ad720078a9f490dd5b5bc8893a926479196e
[2]: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=01a8c731aacbdbed0eb5682d13cc074dc7e25fb3

Signed-off-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20211221054105.178795-2-git@xen0n.name>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

include/elf.h

@@ -182,6 +182,8 @@ typedef struct mips_elf_abiflags_v0 {
 
 #define EM_NANOMIPS     249     /* Wave Computing nanoMIPS */
 
+#define EM_LOONGARCH    258     /* LoongArch */
+
 /*
  * This is an interim value that we will use until the committee comes
  * up with a final number.