[libc++] Add configuration define for off_t functions

[libc++] Add configuration define for off_t functions

Create this define in __config and use it elsewhere, instead of checking
the operating system/library defines in other files. The aim is to
reduce the usage of _WIN32 outside __config. No functional change.

Differential Revision: https://reviews.llvm.org/D25741

llvm-svn: 285582