Make hb_blob_create_from_file more portable (#1027)

Make hb_blob_create_from_file more portable (#1027)

This makes it compatible with ARMCC which I had access in
a collaboration with @imgtec, thanks!

Basically hb_blob_create_from_file features three code paths,
mmap, Win32 and fallback.

We had fallback implementation even before this but it was relied
to "open" which is not available on some environments. This change
improved the situtation by using only fopen and friends for
fallback path.

Interestingly we could use "open" on Windows but in fact it was
emulated by MSVCRT so I've completely split that from Unix path
now that we have a distinct path for fallback path also.