EM_LOONGARCH has been officially registered as e_machine 258.
Add the corresponding definition to elftoolchain's ELF header
and teach readelf/elfdump to recognize and print it.
Upstream registration in GNU binutils:
https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=4cf2ad720078a9f490dd5b5bc8893a926479196e
Signed-off-by: shipujin <shipujin.t@gmail.com>