fix garbage collection race: save stack trace when changing process state to Gsyscall, not after.
fix garbage collection race: save stack trace
when changing process state to Gsyscall, not after.

R=r
DELTA=8  (4 added, 3 deleted, 1 changed)
OCL=30320
CL=30325

gnanderson/go,nightlyone/golang,ssrl/go,gnanderson/go,dsriram/golang,h8liu/golang,pombredanne/go-deleteme,tv42/old-go,ssrl/go,ssrl/go,ssrl/go,apparentlymart/golang,tianon/debian-golang,gnanderson/go,apparentlymart/golang,h8liu/golang,tianon/debian-golang,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,bdwalton/debian-golang,h8liu/golang,pombredanne/go-deleteme,bdwalton/debian-golang,nightlyone/golang,bdwalton/debian-golang,apparentlymart/golang,nightlyone/golang,dsriram/golang,dsriram/golang,bdwalton/debian-golang,apparentlymart/golang,nightlyone/golang,pombredanne/go-deleteme,nightlyone/golang,tv42/old-go,h8liu/golang,h8liu/golang,tianon/debian-golang,bdwalton/debian-golang,apparentlymart/golang,tianon/debian-golang,tv42/old-go,tv42/old-go,gnanderson/go,gnanderson/go,bdwalton/debian-golang,nightlyone/golang,apparentlymart/golang,h8liu/golang,bdwalton/debian-golang,ssrl/go,tianon/debian-golang,bdwalton/debian-golang,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,tianon/debian-golang,tv42/old-go,tianon/debian-golang,tv42/old-go,ssrl/go,tv42/old-go,nightlyone/golang,pombredanne/go-deleteme,tianon/debian-golang,dsriram/golang,ssrl/go,dsriram/golang,pombredanne/go-deleteme,h8liu/golang,apparentlymart/golang