compiler-rt: math functions reorg

* unify the logic for exporting math functions from compiler-rt,
   with the appropriate suffixes and prefixes.
   - add all missing f128 and f80 exports. Functions with missing
     implementations call other functions and have TODO comments.
   - also add f16 functions
 * move math functions from freestanding libc to compiler-rt (#7265)
 * enable all the f128 and f80 code in the stage2 compiler and behavior
   tests (#11161).
 * update std lib to use builtins rather than `std.math`.

lib/std/math/complex.zig

@@ -115,7 +115,7 @@ pub fn Complex(comptime T: type) type {
 
         /// Returns the magnitude of a complex number.
         pub fn magnitude(self: Self) T {
-            return math.sqrt(self.re * self.re + self.im * self.im);
+            return @sqrt(self.re * self.re + self.im * self.im);
         }
     };
 }