stage2: change x86_64 max int alignment from 8 to 16

For x86_64, LLVMABIAlignmentOfType(i128) reports 8. However I think 16
is a better number for two reasons:
1. Better machine code when loading into SIMD register.
2. The C ABI wants 16 for extern structs.

Author: andrewrk

lib/std/target.zig

@@ -1784,7 +1784,6 @@ pub const Target = struct {
             .armeb,
             .thumb,
             .thumbeb,
-            .x86_64,
             .hexagon,
             .mips,
             .mipsel,
@@ -1811,6 +1810,12 @@ pub const Target = struct {
                 .windows => 8,
                 else => 4,
             },
+
+            // For x86_64, LLVMABIAlignmentOfType(i128) reports 8. However I think 16
+            // is a better number because of two reasons:
+            // 1. Better machine code when loading into SIMD register.
+            // 2. The C ABI wants 16 for extern structs.
+            .x86_64,
             .aarch64,
             .aarch64_be,
             .aarch64_32,

test/behavior/align.zig

@@ -55,6 +55,9 @@ test "alignment of struct with pointer has same alignment as usize" {
 }
 
 test "alignment and size of structs with 128-bit fields" {
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
+
     const A = struct {
         x: u128,
     };
@@ -67,7 +70,6 @@ test "alignment and size of structs with 128-bit fields" {
         .armeb,
         .thumb,
         .thumbeb,
-        .x86_64,
         .hexagon,
         .mips,
         .mipsel,
@@ -128,6 +130,7 @@ test "alignment and size of structs with 128-bit fields" {
             },
         },
 
+        .x86_64,
         .aarch64,
         .aarch64_be,
         .aarch64_32,