riscv: module: move find_section to module.h

Move find_section() to module.h so that the implementation can be shared
by the alternatives code. This will allow us to use alternatives in
the vdso.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Link: https://lore.kernel.org/r/20230128172856.3814-8-jszhang@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>