configure, meson.build: Mark support for loongarch64 hosts

configure, meson.build: Mark support for loongarch64 hosts

Example output of `uname -a` on an initial Gentoo LA64 port, running
the upstream submission version of Linux (with some very minor patches
not influencing output here):

> Linux <hostname> 5.14.0-10342-g37a00851b145 #5 SMP PREEMPT Tue Aug 10 12:56:24 PM CST 2021 loongarch64 GNU/Linux

And the same on the vendor-supplied Loongnix 20 system, with an early
in-house port of Linux, and using the old-world ABI:

> Linux <hostname> 4.19.167-rc5.lnd.1-loongson-3 #1 SMP Sat Apr 17 07:32:32 UTC 2021 loongarch64 loongarch64 loongarch64 GNU/Linux

So a name of "loongarch64" matches both, fortunately.

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: <20211221054105.178795-31-git@xen0n.name>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>