riscv: implement VirtIO target

This allows running RISC-V tests in CI using QEMU, which should help
catch bugs.

targets/riscv-qemu.json

@@ -0,0 +1,7 @@
+{
+	"inherits": ["riscv"],
+	"features": ["+a", "+c", "+m"],
+	"build-tags": ["virt", "qemu"],
+	"linkerscript": "targets/riscv-qemu.ld",
+	"emulator": ["qemu-system-riscv32", "-machine", "virt", "-nographic", "-bios", "none", "-kernel"]
+}

targets/riscv-qemu.ld

@@ -0,0 +1,16 @@
+
+/* Memory map:
+ * https://github.com/qemu/qemu/blob/master/hw/riscv/virt.c
+ * RAM and flash are set to 1MB each. That should be enough for the foreseeable
+ * future. QEMU does not seem to limit the flash/RAM size and in fact doesn't
+ * seem to differentiate between it.
+ */
+MEMORY
+{
+    FLASH_TEXT (rw) : ORIGIN = 0x80000000, LENGTH = 0x100000
+    RAM (xrw)       : ORIGIN = 0x80100000, LENGTH = 0x100000
+}
+
+_stack_size = 2K;
+
+INCLUDE "targets/riscv.ld"