riscv64: disable mmu before kernel jump

hi,

i tried to install 7.9 under libvirt/qemu with uefi/edk2 firmware, but
it faults right after the loader tries to jump to the kernel.

it seems since edk2 ~2023, it runs with paging enabled and maps memory
non-executable. to fix this we can disable S mode paging by setting SATP
CSR to 0 before the jump.

linux does the same in its efi stub.

this lets bsd.rd and bsd boot for me in the vm.

diff --git a/sys/arch/riscv64/stand/efiboot/exec.c b/sys/arch/riscv64/stand/efiboot/exec.c
index 0a71ccecccb..ec7325ad777 100644
--- a/sys/arch/riscv64/stand/efiboot/exec.c
+++ b/sys/arch/riscv64/stand/efiboot/exec.c
@@ -83,6 +83,13 @@ run_loadfile(uint64_t *marks, int howto)
 
 	cpu_flush_dcache((vaddr_t)fdt, fdt_get_size(fdt));
 
+	/*
+	 * Firmware may be running with paging enabled and the kernel
+	 * load area mapped non-executable; the kernel expects to be
+	 * entered in bare mode.
+	 */
+	__asm volatile("csrw satp, zero; sfence.vma" ::: "memory");
+
 	(*(startfuncp)(marks[MARK_ENTRY]))((void *)marks[MARK_END], 0, fdt);
 
 	/* NOTREACHED */