explicitly assemble all arm asm sources as UAL

these files are all accepted as legacy arm syntax when producing arm
code, but legacy syntax cannot be used for producing thumb2 with
access to the full ISA. even after switching to UAL, some asm source
files contain instructions which are not valid in thumb mode, so these
will need to be addressed separately.

remove non-working pre-armv4t support from arm asm

the idea of the three-instruction sequence being removed was to be
able to return to thumb code when used on armv4t+ from a thumb caller,
but also to be able to run on armv4 without the bx instruction
available (in which case the low bit of lr would always be 0).
however, without compiler support for generating such a sequence from
C code, which does not exist and which there is unlikely to be
interest in implementing, there is little point in having it in the
asm, and it would likely be easier to add pre-armv4t support via
enhanced linker handling of R_ARM_V4BX than at the compiler level.

removing this code simplifies adding support for building libc in
thumb2-only form (for cortex-m).

fix inconsistent visibility for internal syscall symbols

initial commit of the arm port

this port assumes eabi calling conventions, eabi linux syscall
convention, and presence of the kernel helpers at 0xffff0f?0 needed
for threads support. otherwise it makes very few assumptions, and the
code should work even on armv4 without thumb support, as well as on
systems with thumb interworking. the bits headers declare this a
little endian system, but as far as i can tell the code should work
equally well on big endian.

some small details are probably broken; so far, testing has been
limited to qemu/aboriginal linux.