Also implement go:nosplit annotation. Not really needed
for now, but we'll definitely need it for other conversions.
benchmark old ns/op new ns/op delta
BenchmarkRuneIterate 534 474 -11.24%
BenchmarkRuneIterate2 535 470 -12.15%
LGTM=bradfitz
R=golang-codereviews, dave, bradfitz, minux
CC=golang-codereviews
https://golang.org/cl/93380044
now that cgo2c can handle it,
merge x.c and x_go.cgo into
a single x.cgo, for x=float,malloc,sema.
R=r
DELTA=1950 (954 added, 996 deleted, 0 changed)
OCL=30951
CL=30964