runtime: restructured os1_linux.go, added mips64 support

Linux/mips64 uses a different type of sigset. To deal with it, related
functions in os1_linux.go is refactored to os1_linux_generic.go
(used for non-mips64 architectures), and os1_linux_mips64x.go (only used
in mips64{,le}), to avoid code copying.

Change-Id: I5cadfccd86bfc4b30bf97e12607c3c614903ea4c
Reviewed-on: https://go-review.googlesource.com/14991


Reviewed-by: Ian Lance Taylor <iant@golang.org>