runtime: make Go and C mallocgc signatures match

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://codereview.appspot.com/169920043
misc/cgo/test: fix freebsd test failure by moving test to its own package.

(The assertion depends on a per-package gensym counter whose
value varies based on what else is in the package.)

LGTM=khr
R=khr, rsc
CC=golang-codereviews
https://codereview.appspot.com/169930043
gc: abort if given an unknown debug (-d) flag

The check for unknown command line debug flags in gc was
incorrect: the loop over debugtab terminates when it reaches a
nil entry, but it was only reporting an error if the parser
had passed the last entry of debugtab (which it never did).
Fix this by reporting the usage error if the loop reaches a
nil entry.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/166110043
test: comment out failing cases from sinit.go

One failing case this removes is:

var bytes = []byte("hello, world")
var copy_bytes = bytes

We could handle this in the compiler, but it requires special
case for a variable that is initialized to the value of a
variable that is initialized to a string literal converted to
[]byte.  This seems an unlikely case--it never occurs in the
standrd library--and it seems unnecessary to write the code to
handle it.

If we do want to support this case, one approach is
https://codereview.appspot.com/171840043.

The other failing cases are of the form

var bx bool
var copy_bx = bx

The compiler used to initialize copy_bx to false.  However,
that led to issue 7665, since bx may be initialized in non-Go
code.  The compiler no longer assumes that bx must be false,
so copy_bx can not be statically initialized.

We can fix these with https://codereview.appspot.com/169040043
if we also pass -complete to the compiler as part of this
test.  This is OK but it's too late in the release cycle.

Fixes issue 8746.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/165400043