In preparation for issues such as #10180, Getpagesize() needs to be
fixed to return the actual system's page size instead of assuming it's
always 4096.

This is particularly important for future platform support on Solaris.

Fixes #12076

Change-Id: I78205165909529215fe93ed6ba56e9c3ee1c2abb
Reviewed-on: https://go-review.googlesource.com/14483Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>