runtime: correct various Go -> C function calls

Some things get converted.
Other things (too complex or too many C deps) get onM calls.
Other things (too simple) get #pragma textflag NOSPLIT.

After this CL, the offending function list is basically:
        - panic.c
        - netpoll.goc
        - mem*.c
        - race stuff
        - readgstatus
        - entersyscall/exitsyscall

LGTM=r, iant
R=golang-codereviews, r, iant
CC=dvyukov, golang-codereviews, khr
https://codereview.appspot.com/140930043
runtime: fix sigtrampPC on Windows

The arm5 build breakage at CL 139110043 was caused by
calling funcPC on a lessstack defined as a struct{}.
That symbol ended up with a non-4-aligned address,
which caused the memory fault that broke the builders.
The definition of lessstack was fixed in CL 140880043.

Tracking that down suggested that it would be worth
looking for the same bug elsewhere in the directory.
This is the only one I found.

LGTM=bradfitz
R=golang-codereviews, dave, bradfitz
CC=dvyukov, golang-codereviews, iant, khr, r
https://codereview.appspot.com/134410043