RISC-V: riscv-ext.def: Add allocated group IDs and group bit positions

RISC-V: riscv-ext.def: Add allocated group IDs and group bit positions

The riscv-c-api-doc defines a group ID and and a bit position for some
extension.  Most of them are set in riscv-ext.def, but some are missing
and one bit position (for Zilsd) is wrong.

This patch replaces the `BITMASK_NOT_YET_ALLOCATED` value for the actual
allocated value wherever possible and fixes the bit position for Zilsd.

Currently, we don't have any infrastructure to utilize the information
that is placed into riscv_ext_info_t::m_bitmask_group_id and
riscv_ext_info_t::m_bitmask_group_bit_pos.  This also means we can't
test.

gcc/ChangeLog:

	* config/riscv/riscv-ext.def: Add allocated group IDs and
	group bit positions.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>