Implement custom assembly thunks for hot race calls (memory accesses and function entry/exit).
The thunks extract caller pc, verify that the address is in heap or global and switch to g0 stack.

Before:
ok  	regexp	3.692s
ok  	compress/bzip2	9.461s
ok  	encoding/json	6.380s
After:
ok  	regexp	2.229s (-40%)
ok  	compress/bzip2	4.703s (-50%)
ok  	encoding/json	3.629s (-43%)

For comparison, normal non-race build:
ok  	regexp	0.348s
ok  	compress/bzip2	0.304s
ok  	encoding/json	0.661s
Race build:
ok  	regexp	2.229s (+540%)
ok  	compress/bzip2	4.703s (+1447%)
ok  	encoding/json	3.629s (+449%)

Also removes some race-related special cases from cgocall and scheduler.
In long-term it will allow to remove cyclic runtime/race dependency on cmd/cgo.

Fixes #4249.
Fixes #7460.
Update #6508
Update #6688

R=iant, rsc, bradfitz
CC=golang-codereviews
https://golang.org/cl/55100044