cmd/gc: racewalk: fix a bunch of minor issues

cmd/gc: racewalk: fix a bunch of minor issues
1. Prepend racefuncenter() to fn->enter -- fn->enter can contain new() calls,
and we want them to be in the scope of the function.
2. Dump fn->enter and fn->exit.
3. Add TODO that OTYPESW expression can contain interesting memory accesses.
4. Ignore only _ names instead of all names starting with _.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6822048