riscv: Enable interrupts during syscalls with M-Mode

riscv: Enable interrupts during syscalls with M-Mode

When running is M-Mode (no MMU config), MPIE does not get set. This
results in all syscalls being executed with interrupts disabled as
handle_exception never sets SR_IE as it always sees SR_PIE being
cleared. Fix this by always force enabling interrupts in
handle_syscall when CONFIG_RISCV_M_MODE is enabled.

Signed-off-by: Damien Le Moal <[email protected]>
Reviewed-by: Palmer Dabbelt <[email protected]>
Signed-off-by: Palmer Dabbelt <[email protected]>