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.
build: check if AS supports the '.func' directive

Not supported by Clang's integrated assembler. Since it just adds
debug information it can safely omitted.