ARM IAS: (partially) support .arch_extension directive

This adds a partial implementation of the .arch_extension directive to the
integrated ARM assembler.  There are a number of limitations to this
implementation arising from the target backend support rather than the
implementation itself.  Namely, iWMMXT (v1 and v2), Maverick, and XScale support
is not present in the ARM backend.  Currently, there is no check for A-class
only (needed for virt), and no ARMv6k detection (needed for os and sec).  The
remainder of the extensions are fully supported.