runtime: export cgoCheck functions

    
    The functions cgoCheckPointer and cgoCheckResult are called by code
    generated by cgo. That means that we need to export them using
    go:linkname, as otherwise they are local symbols. The cgo code
    currently uses weak references to only call the symbols if they are
    defined, which is why it has been working--the cgo code has not been
    doing any checks.
    
    Reviewed-on: https://go-review.googlesource.com/80295

From-SVN: r255347

gcc/go/gofrontend/MERGE

-1949a203fca0c8bde6f2690ebc36427c5e3953c7
+338f7434175bb71f3c8905e9ad7f480aec3afee6
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/cgocall.go

@@ -11,6 +11,10 @@ import (
 	"unsafe"
 )
 
+// Functions called by cgo-generated code.
+//go:linkname cgoCheckPointer runtime.cgoCheckPointer
+//go:linkname cgoCheckResult runtime.cgoCheckResult
+
 // Pointer checking for cgo code.
 
 // We want to detect all cases where a program that does not use