runtime: use reflect·call() to enter the function gc()

runtime: use reflect·call() to enter the function gc()

Garbage collection code (to be merged later) is calling functions
which have many local variables. This increases the probability that
the stack capacity won't be big enough to hold the local variables.
So, start gc() on a bigger stack to eliminate a potentially large number
of calls to runtime·morestack().

R=rsc, remyoudompheng, dsymonds, minux.ma, iant, iant
CC=golang-dev
https://golang.org/cl/6846044