arch: arm: aarch32: Allow selecting compiler instruction set

This commit introduces the `COMPILER_ISA_THUMB2` symbol to allow
choosing either the ARM or Thumb instruction set for C code
compilation.

In addition, this commit introduces the `ASSEMBLER_ISA_THUMB2` helper
symbol to specify the default target instruction set for the assembler.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>