riscv tunes: ISA Implementation of RISC-V tune features

riscv tunes: ISA Implementation of RISC-V tune features

This implements the following base ISAs:

* rv32i, rv64i
* rv32e, rv64i

The following ABIs:
* ilp32, ilp32e, ilp32f, ilp32d
* lp64, lp64e, lp64f, lp64d

The following ISA extension are also implemented:
* M - Integer Multiplication and Division Extension
* A - Atomic Memory Extension
* F - Single-Precision Floating-Point Extension
* D - Double-Precision Floating-Point Extension
* C - Compressed Extension
* B - Bit Manipulation Extension (implies Zba, Zbb, Zbs)
* V - Vector Operations Extension
* Zicsr - Control and Status Register Access Extension
* Zifencei - Instruction-Fetch Fence Extension
* Zba - Address bit manipulation extension
* Zbb - Basic bit manipulation extension
* Zbc - Carry-less multiplication extension
* Zbs - Single-bit manipulation extension
* Zicbom - Cache-block management extension

The existing processors tunes are preserved:
* riscv64 (rv64gc)
* riscv32 (rv32gc)
* riscv64nf (rv64imac_zicsr_zifencei)
* riscv32nf (rv32imac_zicsr_zifencei)
* riscv64nc (rv64imafd_zicsr_zifencei)

Previously defined feature 'big-endian' has been removed as it was not used.

Signed-off-by: Mark Hatle <mark.hatle@amd.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>