aarch64: add darwin style PAGE/PAGEOFF relocations
aarch64: remove optional :pg_hi21: for adrp instruction

Clang's integrated assembler does not support it.
aarch64: use '#' for whole line asm comments

Both gnu as and clang treat lines starting with '#' as comments if they
aren't consumed by the C-style preprocessor.
Using '//' does not work with clang since comments are removed before
macro expansion.
arm: check if AS supports .dn

Move the GNU as check before the arch specific asm checks since the .dn
check requires gas compatible assembler.

Disable the VC-1 motion compensation NEON asm which is the only part
using that directive. The integrated assembler in the upcoming clang 3.5
does not support .dn/.qn without plans to change that. Too much effort
to implement it while it is rarely used.

http://llvm.org/bugs/show_bug.cgi?id=18199.
Merge commit 'fd2981ea92d9a776fcb1a13377dce1c8a7db7b5e'

* commit 'fd2981ea92d9a776fcb1a13377dce1c8a7db7b5e':
  aarch64: add darwin style PAGE/PAGEOFF relocations

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit '6a0fa4d86f2b3e9060a1153b39fa3bfe36f1b617'

* commit '6a0fa4d86f2b3e9060a1153b39fa3bfe36f1b617':
  aarch64: remove optional :pg_hi21: for adrp instruction

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit '68a06b3a639ee21c78532ee4c50c3366bf890ff7'

* commit '68a06b3a639ee21c78532ee4c50c3366bf890ff7':
  aarch64: use '#' for whole line asm comments

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit '896a5bff64264f4d01ed98eacc97a67260c1e17e'

* commit '896a5bff64264f4d01ed98eacc97a67260c1e17e':
  arm: check if AS supports .dn

Conflicts:
	configure
	libavcodec/arm/vc1dsp_init_neon.c

Merged-by: Michael Niedermayer <michaelni@gmx.at>
doc/muxers: Document problems of omitting nut syncpoints as well as alternatives

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>