cmd/compile: switch cannedimports to use a Biobuf

Allows eliminating the separate lexer code paths for reading from cp
in the next CL.

Change-Id: I49098ecef32b735c4a01374443c2f847235ff964
Reviewed-on: https://go-review.googlesource.com/19750Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/lex.go

@@ -851,7 +851,7 @@ func importfile(f *Val) {
 func cannedimports(file string, cp string) {
 	lexlineno++ // if sys.6 is included on line 1,
 	pushedio = curio
-	curio = Io{infile: file, cp: cp}
+	curio = Io{infile: file, bin: obj.Binitr(strings.NewReader(cp))}
 	typecheckok = true
 	incannedimport = 1