Fix MinGW-related portability issues.

Create and use FMT* macros that are equivalent to the PRI* macros that
inttypes.h defines.  This allows uniform use of the Unix-specific format
specifiers, e.g. "%zu", as well as avoiding Windows-specific definitions
of e.g. PRIu64.

Add ffs()/ffsl() support for compiling with gcc.

Extract compatibility definitions of ENOENT, EINVAL, EAGAIN, EPERM,
ENOMEM, and ENORANGE into include/msvc_compat/windows_extra.h and
use the file for tests as well as for core jemalloc code.