Correct rationale in simd_x86_64.S comment

The SIMD operations are written in assembly because loading and
operating on indeterminate values is UB in C++ but well-defined in
assembly. msan not tracking the loads is a side effect, not the reason.
Closes #71

simd_x86_64.S

@@ -1,7 +1,8 @@
 // SIMD operations on potentially-indeterminate Node16::index[16] bytes.
-// Written in assembly so msan doesn't track the loads. The caller is
+// Written in assembly because loading and operating on indeterminate values
-// responsible for masking the returned bitfield to [0, numChildren) before
+// is UB in C++ but well-defined in assembly. (A side effect is that msan
-// using it.
+// doesn't track the loads.) The caller is responsible for masking the
+// returned bitfield to [0, numChildren) before using it.
 //
 // All functions return a 16-bit bitmask in %eax (bit i set = condition true
 // at index i). The upper 16 bits of %eax are zero.
@@ -71,4 +72,15 @@ mask_in_range_16:
     ret
     .size mask_in_range_16, .-mask_in_range_16
 
+    .section .note.gnu.property,"a",@note
+    .p2align 3, 0x0
+    .long 4
+    .long 16
+    .long 5
+    .asciz "GNU"
+    .long 0xc0000002
+    .long 4
+    .long 0x3
+    .p2align 3, 0x0
+
     .section .note.GNU-stack,"",@progbits