cgo: implement #cgo CFLAGS

This implementation is still very limited but provides a base to build
upon. Limitations:

  * CGO_CFLAGS etc is not taken into account.
  * These CFLAGS are not used in C files compiled with the package.
  * Other flags (CPPFLAGS, LDFAGS, ...) are not yet implemented.

cgo/testdata/flags.out.go

@@ -0,0 +1,32 @@
+// CGo errors:
+//     testdata/flags.go:5:7: invalid #cgo line: NOFLAGS
+//     testdata/flags.go:8:13: invalid flag: -fdoes-not-exist
+
+package main
+
+import "unsafe"
+
+var _ unsafe.Pointer
+
+const C.BAR = 3
+
+type C.int16_t = int16
+type C.int32_t = int32
+type C.int64_t = int64
+type C.int8_t = int8
+type C.uint16_t = uint16
+type C.uint32_t = uint32
+type C.uint64_t = uint64
+type C.uint8_t = uint8
+type C.uintptr_t = uintptr
+type C.char uint8
+type C.int int32
+type C.long int32
+type C.longlong int64
+type C.schar int8
+type C.short int16
+type C.uchar uint8
+type C.uint uint32
+type C.ulong uint32
+type C.ulonglong uint64
+type C.ushort uint16