Add a binemit test command.

This makes it possible to write file tests that verify the binary
encoding of machine code.

filetests/isa/riscv/binary32.cton

@@ -0,0 +1,13 @@
+; Binary emission of 32-bit code.
+test binemit
+isa riscv
+
+function int32() {
+ebb0:
+    [-,%x5]             v1 = iconst.i32 1
+    [-,%x6]             v2 = iconst.i32 2
+    [R#0c,%x7]          v10 = iadd v1, v2       ; bin: 006283b3
+    [R#200c,%x8]        v11 = isub v1, v2       ; bin: 40628433
+    [R#10c]             v12 = imul v1, v2
+    return
+}