[compiler-rt] Add a prefix on the windows mmap symbols (#78037)

For Windows, the compiler-rt profile library contains a polyfill
reimplementation of the mmap family of functions.

Previously, the runtime library exposed those symbols like, "mmap", in
the user symbol namespace. This could cause misdetections by configure
scripts that check for the "mmap" function just by linking, without
including headers.

Add a prefix on the symbols, and make an undeclared function static.

This fixes such an issue reported at
https://github.com/mstorsjo/llvm-mingw/issues/390.