[dev.ssa] cmd/compile: 386 port now works

[dev.ssa] cmd/compile: 386 port now works

GOARCH=386 SSATEST=1 ./all.bash passes

Caveat: still needs changes to test/ files to use *_ssa.go versions.  I
won't check those changes in with this CL because the builders will
complain as they don't have SSATEST=1.

Mostly minor fixes.

Implement float <-> uint32 in assembly.  It seems the simplest option
for now.

GO386=387 does not work.  That's why I can't make SSA the default for
386 yet.

Change-Id: Ic4d4402104d32bcfb1fd612f5bb6539f9acb8ae0
Reviewed-on: https://go-review.googlesource.com/25119
Reviewed-by: Cherry Zhang <cherryyz@google.com>