arch: arm: cortex_r: Enable Thumb2 instruction set support

The ARMv7-R architecture supports both Thumb-2 (T32) and ARM (A32)
instruction sets.

This commit selects the `ISA_THUMB2` symbol to indicate that the
ARMv7-R architecture supports the Thumb-2 instruction set, which can
be enabled by selecting the `COMPILER_ISA_THUMB2` symbol.

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