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.

compiler/rustc_span/src/symbol.rs

@@ -1101,6 +1101,7 @@ symbols! {
         simd_reduce_or,
         simd_reduce_xor,
         simd_rem,
+        simd_round,
         simd_saturating_add,
         simd_saturating_sub,
         simd_scatter,
@@ -1109,6 +1110,7 @@ symbols! {
         simd_shl,
         simd_shr,
         simd_sub,
+        simd_trunc,
         simd_xor,
         since,
         sinf32,