runtime: use internal/strconv

Runtime doing its own number formatting dates back to
when runtime was the bottom-most Go package.
Those days are long gone. Use internal/strconv to avoid
duplicating code and also to get better floating-point
formatting:

% go1.24.6 run x.go
+1.234568e+004
% go run x.go
12345.678
%

With accurate floating point it becomes necessary to
introduce separate printers for float32 vs float64 and
for complex64 vs complex128. Otherwise float32(93.7)
prints as 93.69999694824219.

Change-Id: I25ae3f09519342dc3d1dcabf4711651423e00128
Reviewed-on: https://go-review.googlesource.com/c/go/+/716002
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: rsc

src/cmd/compile/internal/typecheck/_builtin/runtime.go

@@ -49,11 +49,13 @@ func goPanicSlice3CU(x uint, y int)
 func goPanicSliceConvert(x int, y int)
 
 func printbool(bool)
-func printfloat(float64)
+func printfloat64(float64)
+func printfloat32(float32)
 func printint(int64)
 func printhex(uint64)
 func printuint(uint64)
-func printcomplex(complex128)
+func printcomplex128(complex128)
+func printcomplex64(complex64)
 func printstring(string)
 func printpointer(any)
 func printuintptr(uintptr)