cmd/cgo: accept __uint8_t as the uint8_t type

This works around the NetBSD <stdint.h> which defines the type using
"#define" rather than typedef.

Fixes #30918
Updates #29878

Change-Id: I8998eba52139366ae46762bdad5fcae85f9b4027
Reviewed-on: https://go-review.googlesource.com/c/go/+/168337Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/cgo/gcc.go

@@ -2483,7 +2483,9 @@ func (c *typeConv) Type(dtype dwarf.Type, pos token.Pos) *Type {
 		// representation.
 		if exactWidthIntegerType.MatchString(dt.Name) {
 			sub := c.Type(dt.Type, pos)
-			u := c.exactWidthIntegerTypes[strings.TrimSuffix(dt.Name, "_t")]
+			goname := strings.TrimPrefix(dt.Name, "__")
+			goname = strings.TrimSuffix(goname, "_t")
+			u := c.exactWidthIntegerTypes[goname]
 			if sub.Size != u.Size {
 				fatalf("%s: unexpected size: %d vs. %d – %s", lineno(pos), sub.Size, u.Size, dtype)
 			}
@@ -2630,7 +2632,7 @@ func (c *typeConv) Type(dtype dwarf.Type, pos token.Pos) *Type {
 	return t
 }
 
-var exactWidthIntegerType = regexp.MustCompile(`^u?int(8|16|32|64)_t$`)
+var exactWidthIntegerType = regexp.MustCompile(`^(__)?u?int(8|16|32|64)_t$`)
 
 // isStructUnionClass reports whether the type described by the Go syntax x
 // is a struct, union, or class with a tag.