ARM IAS: support .movsp

.movsp is an ARM unwinding directive that indicates to the unwinder that a
register contains an offset from the current stack pointer.  If the offset is
unspecified, it defaults to zero.
tools: add support for decoding ARM attributes

Enhance the ARM specific parsing support in llvm-readobj to support attributes.
This allows for simpler tests to validate encoding of the build attributes as
specified in the ARM ELF specification.
ARM IAS: support .object_arch

The .object_arch directive indicates an alternative architecture to be specified
in the object file.  The directive does *not* effect the enabled feature bits
for the object file generation.  This is particularly useful when the code
performs runtime detection and would like to indicate a lower architecture as
the requirements than the actual instructions used.