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/compiler.go

@@ -791,6 +791,8 @@ func (c *compilerContext) createPackage(irbuilder llvm.Builder, pkg *ssa.Package
 			// Create the function definition.
 			b := newBuilder(c, irbuilder, member)
 			if _, ok := mathToLLVMMapping[member.RelString(nil)]; ok {
+				// The body of this function (if there is one) is ignored and
+				// replaced with a LLVM intrinsic call.
 				b.defineMathOp()
 				continue
 			}