mips32 + gcc9 -- still broken

[Adrian Chadd]

So, the big list of unknown symbols was my fault! Whoops.

i've gotten further using gcc-6.4 by fixing some of the warnings/issues
that have crept up.

Here's a review for one of them:

https://reviews.freebsd.org/D26504

However, now I've hit:

/usr/local/bin/mips-unknown-freebsd13.0-ld:
/usr/home/adrian/work/freebsd/head-embedded/obj/mips_ap/usr/home/adrian/work/freebsd/head-embedded/src/mips.mips/tmp/usr/lib/libc++.so.1:
undefined reference to `__atomic_fetch_sub_8'
/usr/local/bin/mips-unknown-freebsd13.0-ld:
/usr/home/adrian/work/freebsd/head-embedded/obj/mips_ap/usr/home/adrian/work/freebsd/head-embedded/src/mips.mips/tmp/usr/lib/libc++.so.1:
undefined reference to `__atomic_load_8'
/usr/local/bin/mips-unknown-freebsd13.0-ld:
/usr/home/adrian/work/freebsd/head-embedded/obj/mips_ap/usr/home/adrian/work/freebsd/head-embedded/src/mips.mips/tmp/usr/lib/libc++.so.1:
undefined reference to `__atomic_fetch_add_8'

.. looks like we need some 64 bit atomics now in mips32 for libc++ / devd.



-adrian