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/log.zig

@@ -10,7 +10,7 @@ pub fn log(z: anytype) Complex(@TypeOf(z.re)) {
     const r = cmath.abs(z);
     const phi = cmath.arg(z);
 
-    return Complex(T).init(math.ln(r), phi);
+    return Complex(T).init(@log(r), phi);
 }
 
 const epsilon = 0.0001;