Use little-endian ordering for CLIF vconst immediate

Examining wasm-objdump revealed that it stores SIMD constants in little-endian order, e.g.:

000071 func[2] <test_const>:
 000072: fd 02 01 00 00 00 02 00 00 | v128.const 0x00000001 0x00000002 0x00000003 0x00000004
 00007b: 00 03 00 00 00 04 00 00 00 |
 000084: fd 0d 03                   | i32x4.extract_lane 3
 000087: 0b                         | end

This change avoids confusion by making the CLIF representation use little-endian order as well.

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

@@ -10,4 +10,4 @@ ebb0:
     return v0
 }
 
-; sameln: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 12, 34]
+; sameln: [34, 12, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]