Add filetest for verifying emitted rodata (i.e. test rodata)

cranelift/filetests/filetests/isa/x86/rodata-vconst.clif

@@ -0,0 +1,13 @@
+test rodata
+set enable_simd=true
+set probestack_enabled=false
+target x86_64 haswell
+
+; use baldrdash calling convention here for simplicity (avoids prologue, epilogue)
+function %test_vconst_i32() -> i32x4 baldrdash_system_v {
+ebb0:
+    v0 = vconst.i32x4 0x1234
+    return v0
+}
+
+; sameln: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 12, 34]