Disambiguate between arm and arm64 sources by checking the directory
names.
The arm assembly sources can be assembled on arm64 and vice versa
without any effect since all of the implementation is hidden behind
the HAVE_NEON and HAVE_NEON_AARCH64 defines, but it still is cleaner
to not build extra empty object files than to build all *.S files
on all arm variants. (The iOS project files build all of the arm
assembly files, regardless of the target architecture, since
individual files can't easily be excluded based on the target
architecture there.)
This avoids spurious changes to the targets.mk files when mktargets
is rerun on different platforms and different file systems.
In particular, OS X seems to mostly return files sorted
alphabetically, case insensitively, while they are returned in
a file system specific order on linux.
This makes sure that e.g. "make clean" actually removes
the built exe files, and avoids relinking the exe files each
time make is run when cross compiling from linux to windows.
(Make on windows seems to have an exception that knows to skip
rebuilding the 'h264enc' target if 'h264enc' doesn't exist but
'h264enc.exe' does, but this exception doesn't work on normal
unix make.)
This makes sure that all the dependency libs are built before trying
to link them together, fixing parallel make (which occasionally
failed due to trying to link the encoder or decoder command line
binaries before the libs had been built) and also fixes building an
individual component by e.g. "make h264enc".
