runtime: clean up naming of mcallable functions.

Introduce the mFunction type to represent an mcall/onM-able function.
Name such functions using _m.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://codereview.appspot.com/121320043
debug/dwarf: fix typos in comment for UnspecifiedType

LGTM=iant, bradfitz
R=bradfitz, iant
CC=golang-codereviews
https://codereview.appspot.com/120700043