With this change the compiler emits a bitmap for each function
covering its stack frame arguments area.  If an argument word
is known to contain a pointer, a bit is set.  The garbage
collector reads this information when scanning the stack by
frames and uses it to ignores locations known to not contain a
pointer.

R=golang-dev, bradfitz, daniel.morsing, dvyukov, khr, khr, iant, cshapiro
CC=golang-dev
https://golang.org/cl/9223046