On 64-bit big-endian GNU/Linux machines we need to treat sigset as a
single uint64, not as a pair of uint32 values. This fix was already made
for s390x, but not for ppc64 (which is big-endian--the little endian
version is known as ppc64le). So copy os_linux_390.x to
os_linux_be64.go, and use build constraints as needed.

Fixes #17361

Change-Id: Ia0eb18221a8f5056bf17675fcfeb010407a13fb0
Reviewed-on: https://go-review.googlesource.com/30602
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>