ARM: add explicit .arch and .fpu directives to asm.S

This prevents build errors when compiler and assembler default
targets differ.  Ideally each file would declare the highest
level it requires.  This is however not easily possible as it
complicates assembling pre-armv6t2 code in Thumb-2 mode.

HAVE_NEON is used as indicator for ARMv7-A since no other
symbol exists for this and NEON is only available in this
variant.

Signed-off-by: Mans Rullgard <mans@mansr.com>
