pthread types in C++ builds, attempt 2.
The problem with attempt 1 was making assumptions of what the MD
__cpu_simple_lock_t (declared volatile) looks like. To get a same type
except non-volatile, we change the MD type to __cpu_simple_lock_nv_t
and typedef __cpu_simple_lock_t as a volatile __cpu_simple_lock_nv_t.
IMO, __cpu_simple_lock_t should not be volatile at all, but changing it
now is too risky.
Fixes at least Rumprun w/ gcc 5.1/5.2. Furthermore, the mpd application
(and possibly others) will no longer require NetBSD-specific patches.
Tested: build.sh for i386, Rumprun for x86_64 w/ gcc 5.2.
Based on the patch from Christos in lib/49989.
01d7ebdd80