target/sh4: fix reset when using a kernel and an initrd

When a masked exception happens, the SH4 CPU generates a non-masked
reset exception, which then jumps to the reset vector at address
0xA0000000. While this is emulated correctly in QEMU, this does not
work when using a kernel and initrd as this address then contain an
illegal instruction (and there is no guarantee the kernel and initrd
haven't been overwritten).

Therefore call qemu_system_reset_request to reload the kernel and initrd
and load the program counter to the kernel entry point.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>