RISC-V: add helpers for handling immediates in U-type and I-type pairs

Used together U-type and I-type instructions can for example be used to
generate bigger jumps (i.e. in auipc+jalr pairs) by splitting the value
into an upper immediate (i.e. auipc) and a 12bit immediate (i.e. jalr).

Due to both immediates being considered signed this creates some corner
cases, so add some helper to prevent this from getting duplicated in
different places.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
Link: https://lore.kernel.org/r/20221223221332.4127602-12-heiko@sntech.de
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>