elf: Add RISC-V PSABI ELF header defines

elf: Add RISC-V PSABI ELF header defines

Refer to the RISC-V PSABI specification for details:

- https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md

Cc: Michael Tokarev <mjt@tls.msk.ru>
Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Alistair Francis <Alistair.Francis@wdc.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Michael Clark <mjc@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>