Fix definition and use of target ASFLAGS.

* ArchitectureRules define a variable named HAIKU_ASFLAGS, so other
Jamfiles can append to it.
* But, TARGET_ASFLAGS was used instead in the single place where we
tried to make use of this (the verdex BoardSetup), and in the As rule
* Moreover, the As rule used $(architecture) instead of
$(TARGET_PACKAGING_ARCH) (I suspect a bogus copypaste?). So, it never
actually made use of the flags

With this working, add the proper flags to the BeagleBoard BoardSetup,
so we implement atomic operations the ARMv7 way (no need to syscall).
This helps with compiling, as the As and Cc/Cpp rules now agree on the
defines and the atomic implementation to use.
Don't link the kernel as a shared lib on ARM

We have the same problem as on x86_64: posiiton dependant code isn't
allowed in shared libraries. Since Kernel.so is not used at runtime,
we can use the same hack as on x86_64, and use elfedit to make the
linker think our kernel is a shared library.
ARM Atomics: implement __sync_syncronize

GCC doesn't provide an ARM implementation of it. It's easy to write one
for ARMv6 and above, while older archs will need this implemented as a
syscall just like other atomics.