tsan: round Go shadow to page boundary

There is a corner case reported in Go issue tracker:
https://github.com/golang/go/issues/17065
On darwin data/bss segments may not be aligned to page bounary
and mmap seems to be behaving differently than on linux
(shrinks instead of enlarge unaligned regions).

Explicitly round shadow to page bounary before mapping
to avoid any such problems.

llvm-svn: 285454