apparent pmap bug while building lang/mono6

[coypu%sdf.org@localhost]

hi folks,

I have patched lang/mono6 so it can build a working mini mono on
aarch64.
unfortunately, it's *incredibly* crashy. according to Mono's own
reporting, it is hitting (mostly) SIGILLs, sometimes with all the
memory surrounding the instruction pointer being zero.

Mono itself has good aarch64 support for other OSes. Given that node.js
also has issues, I expect it to be a kernel issue. They both utilize a
JIT.

Hopefully having an easy way to reproduce it that produces core dumps
helps.