[libc] Canonicalize generated fenv.h (#127363)

This removes the custom template for fenv.h by declaring all the
standard-specified macros using macro_header.

Author: frobtech

libc/include/fenv.h.def

@@ -1,11 +1,32 @@
 header: fenv.h
-header_template: fenv.h.def
-macros: []
+standards:
+  - stdc
+macros:
+  - macro_name: FE_ALL_EXCEPT
+    macro_header: fenv-macros.h
+  - macro_name: FE_DIVBYZERO
+    macro_header: fenv-macros.h
+  - macro_name: FE_INEXACT
+    macro_header: fenv-macros.h
+  - macro_name: FE_INVALID
+    macro_header: fenv-macros.h
+  - macro_name: FE_OVERFLOW
+    macro_header: fenv-macros.h
+  - macro_name: FE_UNDERFLOW
+    macro_header: fenv-macros.h
+  - macro_name: FE_DOWNWARD
+    macro_header: fenv-macros.h
+  - macro_name: FE_TONEAREST
+    macro_header: fenv-macros.h
+  - macro_name: FE_TOWARDZERO
+    macro_header: fenv-macros.h
+  - macro_name: FE_UPWARD
+    macro_header: fenv-macros.h
+  - macro_name: FE_DFL_ENV
+    macro_header: fenv-macros.h
 types:
   - type_name: fenv_t
   - type_name: fexcept_t
-enums: []
-objects: []
 functions:
   - name: feclearexcept
     standards:
@@ -15,14 +36,14 @@ functions:
       - type: int
   - name: fedisableexcept
     standards:
-      - GNUExtensions
+      - gnu
     return_type: int
     arguments:
       - type: int
     guard: null
   - name: feenableexcept
     standards:
-      - GNUExtensions
+      - gnu
     return_type: int
     arguments:
       - type: int
@@ -35,7 +56,7 @@ functions:
       - type: fenv_t *
   - name: fegetexcept
     standards:
-      - GNUExtensions
+      - gnu
     return_type: int
     arguments: []
   - name: fegetexceptflag