unix: use int8 for RawSockaddrUnix.Path on linux/riscv64

On linux/riscv64, char defaults to unsigned, so explicitly make
RawSockaddrUnix.Path signed.

Patch by Ryan O'Leary.

Updates golang/go#27532

Change-Id: If99a3953462ad98f61d8fafcda3ebb71b44e6018
Reviewed-on: https://go-review.googlesource.com/c/157900
Run-TryBot: Tobias Klauser <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: tklauser

Diff for: unix/linux/types.go

@@ -230,8 +230,8 @@ struct sockaddr_rc {
 // copied from /usr/include/linux/un.h
 struct my_sockaddr_un {
 	sa_family_t sun_family;
-#if defined(__ARM_EABI__) || defined(__powerpc64__)
-	// on ARM char is by default unsigned
+#if defined(__ARM_EABI__) || defined(__powerpc64__) || defined(__riscv)
+	// on some platforms char is unsigned by default
 	signed char sun_path[108];
 #else
 	char sun_path[108];