Sema: clean up zirUnaryMath

* pass air_tag instead of zir_tag
 * also pass eval function so that the branch only happens once and the
   body of zirUnaryMath is simplified
 * Value.sqrt: update to handle f80 and f128 in the normalized way that
   includes handling c_longdouble.

Semi-related change: fix incorrect sqrt builtin name for f80 in stage1.

src/stage1/codegen.cpp

@@ -6996,7 +6996,7 @@ static LLVMValueRef ir_render_soft_f80_float_op(CodeGen *g, Stage1Air *executabl
     const char *func_name;
     switch (instruction->fn_id) {
         case BuiltinFnIdSqrt:
-            func_name = "__sqrt";
+            func_name = "__sqrtx";
             break;
         case BuiltinFnIdSin:
             func_name = "__sinx";