Add more SIMD math.h intrinsics

LLVM supports many functions from math.h in its IR. Many of these have
single-instruction variants on various platforms. So, let's add them so
std::arch can use them.

Yes, exact comparison is intentional: rounding must always return a
valid integer-equal value, except for inf/NAN.

src/intrinsics/simd.rs

@@ -277,5 +277,7 @@ pub(super) fn codegen_simd_intrinsic_call<'tcx>(
         // simd_select
         // simd_rem
         // simd_neg
+        // simd_trunc
+        // simd_floor
     }
 }