Perf changes on linux-amd64-perf by runtime: avoid race with forcegc helper

[bui...@golang.org]

Change 9227fb9e97fd caused perf changes on linux-amd64-perf:

runtime: avoid race with forcegc helper

While we are here, give the gc helper a real function name
that will appear in stack traces.

LGTM=rlh
R=rlh
CC=dvyukov, golang-codereviews
https://codereview.appspot.com/133470043

http://code.google.com/p/go/source/detail?r=9227fb9e97fd

json-1 old new delta
cputime 161130080 167770480 +4.12
time 160999883 167771491 +4.21

json-2 old new delta
cputime 162690170 169290580 +4.06
time 81545728 84752832 +3.93

json-4 old new delta
cputime 164470280 171070695 +4.01
time 41671434 43126693 +3.49

json-8 old new delta
cputime 175898992 182587412 +3.80
time 22202002 23059261 +3.86

http://build.golang.org/perfdetail?commit=9227fb9e97fdaddfab9a43aa3894178c14284e6b&commit0=70fadda83be4173cb2f6f821179ae02c6138de96&kind=builder&builder=linux-amd64-perf