Building Clang Compiler-RT on Windows with MinGW

I had a problem while trying to build Clang with Compiler-RT on Windows with
MinGW. The problem apeared while getting OS memory page size. MinGW, AFAIK,
does not implement sysconf function and does not define _SC_PAGESIZE. The
workaround I've found is to add check, if Compiler-RT is being built on
windows, and add platform dependent code to get the satisfied memory page
size. After my changes everything was succesfuly built. More information in
patch file. enable_execute_stack.c.patch

I guess it's important: I've used mman-win32
<> implementation, 'cause I didn't
find any in MinGW.

P.S. Sorry, if I'm doing something wrong and for my bad English. I'll be
glad for any information about my misstakes.