xen: add files needed for minimal riscv build

Add arch-specific makefiles and configs needed to build for
riscv. Also add a minimal head.S that is a simple infinite loop.
head.o can be built with

$ make XEN_TARGET_ARCH=riscv64 SUBSYSTEMS=xen -C xen tiny64_defconfig
$ make XEN_TARGET_ARCH=riscv64 SUBSYSTEMS=xen -C xen TARGET=riscv64/head.o

No other TARGET is supported at the moment.

Signed-off-by: Connor Davis <connojdavis@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Acked-by: Bobby Eshleman <bobbyeshleman@gmail.com>

config/riscv64.mk

@@ -0,0 +1,5 @@
+CONFIG_RISCV := y
+CONFIG_RISCV_64 := y
+CONFIG_RISCV_$(XEN_OS) := y
+
+CONFIG_XEN_INSTALL_SUFFIX :=