util/riscvtools: Provide a tohost/fromhost symbols so Spike doesn't hang

See https://github.com/riscv/riscv-isa-sim/issues/54 for more
information.

Change-Id: I8cda8dc07866d395eb3ce5d94df8232840fa8b82
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/15288
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

util/riscvtools/spike-elf.ld

@@ -8,4 +8,8 @@ SECTIONS
 	.data : {
 		*(.data)
 	}
+
+	tohost = .;
+	. = . + 8;
+	fromhost = .;
 }