runtime: cache unrolled GC bitmask
Cache unrolled GC bitmask for types up to 64/32K on 64/32-bit systems,
this corresponds to up to 4K cached bitmask.
Perf builders say that 2% of time is spent in unrollgcproginplace_m/unrollgcprog1
on http benchmark:
http://goperfd.appspot.com/log/f42045f45bf61a0da53b724a7c8567824a0ad6c9

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews, khr, rlh
https://codereview.appspot.com/122680043
runtime: fix races on mheap.allspans

This is based on the crash dump provided by Alan
and on mental experiments:

sweep 0 74
fatal error: gc: unswept span
runtime stack:
runtime.throw(0x9df60d)
markroot(0xc208002000, 0x3)
runtime.parfordo(0xc208002000)
runtime.gchelper()

I think that when we moved all stacks into heap,
we introduced a bunch of bad data races. This was later
worsened by parallel stack shrinking.

Observation 1: exitsyscall can allocate a stack from heap at any time (including during STW).
Observation 2: parallel stack shrinking can (surprisingly) grow heap during marking.
Consider that we steadily grow stacks of a number of goroutines from 8K to 16K.
And during GC they all can be shrunk back to 8K. Shrinking will allocate lots of 8K
stacks, and we do not necessary have that many in heap at this moment. So shrinking
can grow heap as well.

Consequence: any access to mheap.allspans in GC (and otherwise) must take heap lock.
This is not true in several places.

Fix this by protecting accesses to mheap.allspans and introducing allspans cache for marking,
similar to what we use for sweeping.

LGTM=rsc
R=golang-codereviews, rsc
CC=adonovan, golang-codereviews, khr, rlh
https://codereview.appspot.com/126510043