Previously, the flags argument to mallocgc was an int in Go,
but a uint32 in C.  Change the Go type to use uint32 so these
agree.  The largest flag value is 2 (and of course no flag
values are negative), so this won't change anything on little
endian architectures, but it matters on big endian.

LGTM=rsc
R=khr, rsc
CC=golang-codereviews
https://golang.org/cl/169920043