[dev.power64] runtime: fix early GC of Defer objects

go_bootstrap was panicking during runtime initialization
(under runtime.main) because Defer objects were being
prematurely GC'd.  This happened because of an incorrect
change to runtime·unrollgcprog_m to make it endian-agnostic
during the conversion of runtime bitmaps to byte arrays.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/161960044