runtime: avoid runtime·cgocall in functions called by forkAndExecInChild
runtime: avoid runtime·cgocall in functions called by forkAndExecInChild

Calling runtime·cgocall could trigger a GC in the child while
gclock was held by the parent.

Fixes issue 7511

LGTM=bradfitz, dvyukov, dave
R=golang-codereviews, bradfitz, dvyukov, dave
CC=golang-codereviews, rsc
https://codereview.appspot.com/75210044

Committer: Dave Cheney <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@cheney.net>

tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go