cmd/cgo, cmd/dist, cmd/go: cgo with clang fixes

1. Workaround the smart clang diagnostics with -Qunused-arguments:
clang: error: argument unused during compilation: '-XXX'
2. if "clang -print-libgcc-file-name" returns non-absolute path, don't
provide that on linker command line.
3. Fix dwarf.PtrType.Size() in cmd/cgo as clang doesn't generate
DW_AT_byte_size for pointer types.
4. Workaround warnings for -Wno-unneeded-internal-declaration with
-Wno-unknown-warning-option.
5. Add -Wno-unused-function.
6. enable race detector test on darwin with clang
(at least Apple clang version 1.7 (tags/Apple/clang-77) works).

Requires CL 7354043.

Update #4829
This should fix most parts of the problem, but one glitch still remains.
DWARF generated by newer clang doesn't differentiate these
two function types:
    void *malloc(size_t);
    void *malloc(unsigned long int);
so you might need to do this to make make.bash pass:
sed -i -e 's/C.malloc(C.size_t/C.malloc(C.ulong/' pkg/os/user/lookup_unix.go

R=golang-dev, dave, iant, rsc
CC=golang-dev
https://golang.org/cl/7351044

src/cmd/cgo/gcc.go

@@ -783,7 +783,13 @@ func (p *Package) gccCmd() []string {
 	if strings.Contains(p.gccName(), "clang") {
 		c = append(c,
 			"-ferror-limit=0",
+			// Apple clang version 1.7 (tags/Apple/clang-77) (based on LLVM 2.9svn)
+			// doesn't have -Wno-unneeded-internal-declaration, so we need yet another
+			// flag to disable the warning. Yes, really good diagnostics, clang.
+			"-Wno-unknown-warning-option",
 			"-Wno-unneeded-internal-declaration",
+			"-Wno-unused-function",
+			"-Qunused-arguments",
 		)
 	}
 
@@ -1049,6 +1055,12 @@ func (c *typeConv) Type(dtype dwarf.Type, pos token.Pos) *Type {
 		return t
 	}
 
+	// clang won't generate DW_AT_byte_size for pointer types,
+	// so we have to fix it here.
+	if dt, ok := base(dtype).(*dwarf.PtrType); ok && dt.ByteSize == -1 {
+		dt.ByteSize = c.ptrSize
+	}
+
 	t := new(Type)
 	t.Size = dtype.Size()
 	t.Align = -1