Fix the indentation of a line in targets.mk

This would be avoided if the targets.mk files are updated by
rerunning mktargets.sh instead of manually updating them.
Rename the makefile variable for arm assembly sources

This is in preparation for adding support for the aarc64 assembly
files as well.
Add support for arm64 assembly source files in mktargets.py

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.)
Exclude the new arm64 include file
Regenerate makefiles to include the new arm64 assembly files
Add support for building the arm64 assembly in platform-arch.mk
Merge pull request #737 from mstorsjo/make-aarch64

Add support for building the arm64 assembly with the make build system