cranelift: Merge all run tests into runtests dir

With this change we now reuse tests across multiple arches.

Duplicate tests were merged into the same file where possible.
Some legacy x86 tests were left in separate files due to incompatibilities with the rest of the test suite.

cranelift/filetests/filetests/runtests/i128-const.clif

@@ -0,0 +1,12 @@
+test run
+target aarch64
+; target s390x TODO: Not yet implemented on s390x
+target x86_64 machinst
+
+function %i128_const_0() -> i64, i64 {
+block0:
+    v1 = iconst.i128 0
+    v2, v3 = isplit v1
+    return v2, v3
+}
+; run: %i128_const_0() == [0, 0]