Depending on flags&KindGCProg,
gc[0] and gc[1] are either pointers or inlined bitmap bits.
That's not compatible with a precise garbage collector:
it needs to be always pointers or never pointers.

Change the inlined bitmap case to store a pointer to an
out-of-line bitmap in gc[0]. The out-of-line bitmaps are
dedup'ed, so that for example all pointer types share the
same out-of-line bitmap.

Fixes #8864.

LGTM=r
R=golang-codereviews, dvyukov, r
CC=golang-codereviews, iant, khr, rlh
https://golang.org/cl/155820043