Preprocessor: improve ACLE 6.4.1, 6.4.2 support

This improves conformance with ACLE 6.4.1.  Define additional macros that
indicate support for the ARM and Thumb instruction set architecture.  This
includes the following set of macros:

  __ARM_ARCH
  __ARM_ARCH_ISA_ARM
  __ARM_ARCH_ISA_THUMB
  __ARM_32BIT_STATE

These help identify the environment that the code is intended to execute on.

Adjust the handling for ACLE 6.4.2 to be more correct.  We would define the
profile as a free-standing token rather than a quoted single character.