[libc] Make LIBC_MATH_NO_ERRNO imply -fno-math-errno (#125794)

This partially reverts #124200. Rather than using a CMake option to
control whether to enable `-fno-math-errno`, use LIBC_MATH_NO_ERRNO
configuration option. While there might be other cases when we want to
set `-fno-math-errno`, having LIBC_MATH_NO_ERRNO imply it should be
always safe and represents a reasonable starting point.

Author: petrhosek

libc/cmake/modules/LLVMLibCCompileOptionRules.cmake

@@ -26,13 +26,6 @@ function(_get_compile_options_from_flags output_var)
         list(APPEND compile_options "-mavx2")
         list(APPEND compile_options "-mfma")
       endif()
-      # For clang, we will build the math functions with `-fno-math-errno` so that
-      # __builtin_fma* will generate the fused-mutliply-add instructions.  We
-      # don't put the control flag to the public config yet, and see if it makes
-      # sense to just enable this flag by default.
-      if(LIBC_ADD_FNO_MATH_ERRNO)
-        list(APPEND compile_options "-fno-math-errno")
-      endif()
     endif()
     if(ADD_ROUND_OPT_FLAG)
       if(LIBC_TARGET_ARCHITECTURE_IS_X86_64)
@@ -102,6 +95,9 @@ function(_get_compile_options_from_config output_var)
 
   if(LIBC_CONF_MATH_OPTIMIZATIONS)
     list(APPEND config_options "-DLIBC_MATH=${LIBC_CONF_MATH_OPTIMIZATIONS}")
+    if(LIBC_CONF_MATH_OPTIMIZATIONS MATCHES "LIBC_MATH_NO_ERRNO")
+      list(APPEND config_options "-fno-math-errno")
+    endif()
   endif()
 
   if(LIBC_CONF_ERRNO_MODE)

libc/cmake/modules/LLVMLibCTestRules.cmake

@@ -3,11 +3,6 @@ function(_get_common_test_compile_options output_var c_test flags)
   _get_compile_options_from_config(config_flags)
   _get_compile_options_from_arch(arch_flags)
 
-  # Remove -fno-math-errno if it was added.
-  if(LIBC_ADD_FNO_MATH_ERRNO)
-    list(REMOVE_ITEM compile_flags "-fno-math-errno")
-  endif()
-
   # Death test executor is only available in Linux for now.
   if(NOT ${LIBC_TARGET_OS} STREQUAL "linux")
     list(REMOVE_ITEM config_flags "-DLIBC_ADD_NULL_CHECKS")

libc/config/baremetal/config.json

@@ -36,7 +36,7 @@
   },
   "math": {
     "LIBC_CONF_MATH_OPTIMIZATIONS": {
-      "value": "(LIBC_MATH_SKIP_ACCURATE_PASS | LIBC_MATH_SMALL_TABLES)"
+      "value": "(LIBC_MATH_SKIP_ACCURATE_PASS | LIBC_MATH_SMALL_TABLES | LIBC_MATH_NO_ERRNO)"
     }
   },
   "general": {