runtime: don't acquirem on malloc fast path
Mallocgc must be atomic wrt GC, but for performance reasons
don't acquirem/releasem on fast path. The code does not have
split stack checks, so it can't be preempted by GC.
Functions like roundup/add are inlined. And onM/racemalloc are nosplit.
Also add debug code that checks these assumptions.

benchmark                     old ns/op     new ns/op     delta
BenchmarkMalloc8              20.5          17.2          -16.10%
BenchmarkMalloc16             29.5          27.0          -8.47%
BenchmarkMallocTypeInfo8      31.5          27.6          -12.38%
BenchmarkMallocTypeInfo16     34.7          30.9          -10.95%

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, rlh, rsc
https://codereview.appspot.com/123100043
runtime: fix dump of data/bss
Fixes issue 8530.

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, rsc
https://codereview.appspot.com/124440043
runtime: implement transfer cache
Currently we do the following dance after sweeping a span:
1. lock mcentral
2. remove the span from a list
3. unlock mcentral
4. unmark span
5. lock mheap
6. insert the span into heap
7. unlock mheap
8. lock mcentral
9. observe empty list
10. unlock mcentral
11. lock mheap
12. grab the span
13. unlock mheap
14. mark span
15. lock mcentral
16. insert the span into empty list
17. unlock mcentral

This change short-circuits this sequence to nothing,
that is, we just cache and use the span after sweeping.

This gives us functionality similar (even better) to tcmalloc's transfer cache.

benchmark            old ns/op     new ns/op     delta
BenchmarkMalloc8     22.2          19.5          -12.16%
BenchmarkMalloc16    31.0          26.6          -14.19%

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, rlh, rsc
https://codereview.appspot.com/119550043