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
http://codereview.appspot.com/5581055

apparentlymart/golang,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,dsriram/golang,dsriram/golang,gnanderson/go,gnanderson/go,tv42/old-go,dsriram/golang,gnanderson/go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,dsriram/golang