cipher: Take care of ENABLE_NEON_SUPPORT.

* cipher/salsa20.c (USE_ARM_NEON_ASM): Define only if
ENABLE_NEON_SUPPORT is defined.
* cipher/serpent.c (USE_NEON): Ditto.
* cipher/sha512.c (USE_ARM_NEON_ASM): Ditto.

The generic C source files must only include NEON support if that is
enabled. The dedicated ASM files are conditionally compiled and thus
do not need to use it.

(cherry picked from commit 52f7c48c901a3de51bd690a218f3de2f71e8d790)

Resolved conflicts:

• cipher/sha1.c: Remove changes - no asm support in 1.6.

• GnuPG-bug-id: T1603
• Signed-off-by: Werner Koch <wk@gnupg.org>