riscv: replace deprecated scall with ecall

scall is a deprecated alias for ecall. ecall is used in several places,
so there is no assembler compatibility concern.

Signed-off-by: Fangrui Song <maskray@google.com>
Link: https://lore.kernel.org/r/20230423223210.126948-1-maskray@google.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>