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.

builder/picolibc.go

@@ -28,6 +28,8 @@ var Picolibc = Library{
 			"-DHAVE_ALIAS_ATTRIBUTE",
 			"-DTINY_STDIO",
 			"-D_IEEE_LIBM",
+			"-D__OBSOLETE_MATH_FLOAT=1", // use old math code that doesn't expect a FPU
+			"-D__OBSOLETE_MATH_DOUBLE=0",
 			"-nostdlibinc",
 			"-isystem", newlibDir + "/libc/include",
 			"-I" + newlibDir + "/libc/tinystdio",
@@ -325,6 +327,24 @@ var picolibcSources = []string{
 	"libm/common/s_scalbln.c",
 	"libm/common/s_signbit.c",
 	"libm/common/s_trunc.c",
+	"libm/common/exp.c",
+	"libm/common/exp2.c",
+	"libm/common/exp_data.c",
+	"libm/common/math_err_with_errno.c",
+	"libm/common/math_err_xflow.c",
+	"libm/common/math_err_uflow.c",
+	"libm/common/math_err_oflow.c",
+	"libm/common/math_err_divzero.c",
+	"libm/common/math_err_invalid.c",
+	"libm/common/math_err_may_uflow.c",
+	"libm/common/math_err_check_uflow.c",
+	"libm/common/math_err_check_oflow.c",
+	"libm/common/log.c",
+	"libm/common/log_data.c",
+	"libm/common/log2.c",
+	"libm/common/log2_data.c",
+	"libm/common/pow.c",
+	"libm/common/pow_log_data.c",
 
 	"libm/math/e_acos.c",
 	"libm/math/e_acosh.c",