kernel:
. modules can be as big as the space (8MB) between them
instead of 4MB; memory is slightly bigger with DBG=-g
arm ucontext:
. r4 is clobbered by the restore function, as it's
used as a scratch register, causing problems for the
DBG=-g build
. r1-r3 are safe for scratch registers, as they are
caller-save, so use r3 instead; and don't bother
restoring r1-r3, but preserve r4