runtime: convert markallocated from C to Go

benchmark                      old ns/op     new ns/op     delta
BenchmarkMalloc8               28.7          22.4          -21.95%
BenchmarkMalloc16              44.8          33.8          -24.55%
BenchmarkMallocTypeInfo8       49.0          32.9          -32.86%
BenchmarkMallocTypeInfo16      46.7          35.8          -23.34%
BenchmarkMallocLargeStruct     907           901           -0.66%
BenchmarkGobDecode             13235542      12036851      -9.06%
BenchmarkGobEncode             10639699      9539155       -10.34%
BenchmarkJSONEncode            25193036      21898922      -13.08%
BenchmarkJSONDecode            96104044      89464904      -6.91%

Fixes issue 8452.

LGTM=khr
R=golang-codereviews, bradfitz, rsc, dave, khr
CC=golang-codereviews
https://codereview.appspot.com/122090043