runtime: fix mkasmh.h

runtime: fix mkasmh.h

We weren't properly deleting the various header
files (that were temporarily renamed) if a $CC
for the current $GOARCH didn't exist.  And since
the compiler checks the current directory for
headers before any -I arguments, this had the
unfortunate side effect of including the last
generated headers instead of the correct ones.

R=r, rsc
CC=golang-dev
https://golang.org/cl/5581055