Add blowfish/serpent ARM assembly files to Makefile.am

* cipher/Makefile.am: Add 'blowfish-arm.S' and 'serpent-armv7-neon.S'.
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Fix for bug https://bugs.g10code.com/gnupg/issue1584

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>