runtime: renamed os2_linux.go to os2_linux_generic.go, added mips64 support

Linux/mips64 has a different sigset type and some different constants.
os2_linux.go is renamed to os2_linux_generic.go, and not used in mips64.
The corresponding file os2_linux_mips64x.go is added.

Change-Id: Ief83845a2779f7fe048d236d3c7da52b627ab533
Reviewed-on: https://go-review.googlesource.com/14992


Reviewed-by: Minux Ma <minux@golang.org>