[base] Update FT_MulFix inlining.
Resolves inconsistencies in 64-bit multiplication discussed in !355.
Importantly, FT_MulFix arguments and return value is FT_Long,
whatever sizeof FT_Long is on 64-bit platforms: 8 bytes on Linux or
4 bytes on Windows.
* include/freetype/internal/ftcalc.h (FT_MulFix_x86_64): Removed.
(FT_MulFix_64): Generalize and prioritize the inline implementation
for all 64-bit platforms ifdef FT_INT64.
* src/base/ftcalc.c (FT_MulFix)[FT_INT64]: Call 'FT_MulFix_64'.
* src/base/ftbase.c: Include 'ftcalc.c' after the FT_MulFix callers
to enable its inlining.
