For KMP_PAGE_SIZE, use getpagesize() on Unix, GetSystemInfo() on Windows

Summary:
The kmp_os.h header is defining the `PAGE_SIZE` macro unconditionally,
even while it is only used directly after its definition, for the
Windows implementation of the `KMP_GET_PAGE_SIZE()` macro.

On at least FreeBSD, but likely all other BSDs too, this macro conflicts
with the one defined in system headers, so remove it, since nothing else
uses it.  Make all Unixes use `getpagesize()` instead, and use
`GetSystemInfo()` for the Windows case.

Reviewers: jlpeyton, jcownie, emaste, AndreyChurbanov

Reviewed By: AndreyChurbanov

Subscribers: AndreyChurbanov, hfinkel, zturner

Differential Revision: https://reviews.llvm.org/D35072

llvm-svn: 308355