riscv: set -march and -mabi based on the Kconfig configuration

Use the new Kconfig entries to construct the ISA string for the -march
compiler flag. The -mabi compiler flag is selected based on the base
integer instruction set.

With this change, the C (compressed instructions) ISA extension is now
enabled for all boards with CONFIG_RISCV_ISA_C set. Buildman reports a
decrease in binary size of 71590 bytes.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>