runtime: fix data/bss shadow memory mapping for race detector
Fixes issue 5175.
Race detector runtime expects values passed to MapShadow() to be page-aligned,
because they are used in mmap() call. If they are not aligned mmap() trims
either beginning or end of the mapping.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8325043

Committer: Andrew Gerrand <adg@golang.org>