libcrypto: Refactor Makefile.asm so it can be run outside of buildenv

Currently Makefile.asm relies on the current buildenv to set CFLAGS
for i386. The current approach also leaves various temporary *.s
files around in the current directory. To make this a bit better:

• Instead of using CFLAGS from buildenv for i386, define the actual flags the perl scripts need: -DOPENSSL_IA32_SSE2 to enable SSE2.

• Change i386 to have the perl scripts write to /dev/stdout to avoid creating temporaries. Previously i386 was generating the temporary files in the OpenSSL contrib src.

• Cleanup temporary *.s files in the all target after generating the real *.S files for architectures which need them.

• Remove a duplicate rule for aes-armv4.S.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D41589