runtime: fix riscv64 sigaction mask field offset

The Linux kernel for riscv64 does not include an sa_restorer field on
its sigaction struct, and expects sa_mask to come immediately after the
sa_flags field. Arrange the fields of the sigaction struct that are
known to the kernel so they appear at the correct byte offsets, and so
they agree with the output of "go tool cgo -godefs".

Follow the example set by the mips/mipsle port to leave the sa_restorer
field in place, but at an offset where it won't hurt anything.

Fixes #49709

Change-Id: I9bb0d7dbd7439d07e3a204461c7d790f33fd4977
Reviewed-on: https://go-review.googlesource.com/c/go/+/367635


Run-TryBot: Rhys Hiltner <rhys@justin.tv>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
Trust: Ian Lance Taylor <iant@golang.org>