compiler: replace math aliases with intrinsics

This is really a few more-or-less separate changes:

  * Remove all math aliases that were used in Go 1.16 and below (the
    math.[A-Z] aliases).
  * Replace math aliases with an assembly implementation (the math.arch*
    aliases) with a LLVM intrinsic, where one is available.
  * Include missing math functions in picolibc build.

This leaves just four math aliases:

  * math.archHypot and math.archModf do not have a LLVM builtin
    equivalent. They could be replaced with calls to libm, and I think
    that would be a good idea in the long term.
  * math.archMax and math.archMin do have a LLVM builtin equivalent
    (llvm.maximum.f64, llvm.minimum.f64), but unfortunately they crash
    when used. Apparently these exact operations are not yet widely
    supported in hardware and they don't have a libm equivalent either.

There are more LLVM builtins that we could use for the math package
(such as FMA), but I will leave that to a future change. It could
potentially speed up some math operations.

compiler/intrinsics.go

@@ -81,9 +81,12 @@ func (b *builder) createMemoryZeroImpl() {
 }
 
 var mathToLLVMMapping = map[string]string{
-	"math.Sqrt":  "llvm.sqrt.f64",
-	"math.Floor": "llvm.floor.f64",
 	"math.Ceil":  "llvm.ceil.f64",
+	"math.Exp":   "llvm.exp.f64",
+	"math.Exp2":  "llvm.exp2.f64",
+	"math.Floor": "llvm.floor.f64",
+	"math.Log":   "llvm.log.f64",
+	"math.Sqrt":  "llvm.sqrt.f64",
 	"math.Trunc": "llvm.trunc.f64",
 }