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 <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>