[libc] Share hdrgen declarations between stdlib.h and malloc.h (#127278)

This uses the new merge_yaml_files feature in hdrgen to share the
source of truth for the malloc suite of functions declared in
both stdlib.h and in malloc.h (without either header including
the other).  It also modernizes the malloc.yaml definition a bit,
including dropping the custom template malloc.h.def file in favor
of using the explicit macros list to generate the includes.

Author: frobtech

libc/include/malloc.h.def

@@ -1,13 +1,17 @@
 header: malloc.h
-header_template: malloc.h.def
-macros: []
-types: []
-enums: []
-objects: []
+standards:
+  - gnu
+merge_yaml_files:
+  - stdlib-malloc.yaml
+macros:
+  - macro_name: M_PURGE
+    macro_header: malloc-macros.h
+  - macro_name: M_PURGE_ALL
+    macro_header: malloc-macros.h
 functions:
   - name: mallopt
     standards:
-      - GNUExtensions
+      - gnu
     return_type: int
     arguments:
       - type: int

libc/include/malloc.yaml

@@ -0,0 +1,44 @@
+# This file has declarations that appear both in <stdlib.h> and in <malloc.h>.
+
+functions:
+  - name: aligned_alloc
+    standards:
+      - stdc
+    return_type: void *
+    arguments:
+      - type: size_t
+      - type: size_t
+  - name: calloc
+    standards:
+      - stdc
+    return_type: void *
+    arguments:
+      - type: size_t
+      - type: size_t
+  - name: free
+    standards:
+      - stdc
+    return_type: void
+    arguments:
+      - type: void *
+  - name: malloc
+    standards:
+      - stdc
+    return_type: void *
+    arguments:
+      - type: size_t
+  - name: realloc
+    standards:
+      - stdc
+    return_type: void *
+    arguments:
+      - type: void *
+      - type: size_t
+
+# Note: glibc's <stdlib.h> and <malloc.h> both have these, which are
+# currently missing here:
+#  - name: reallocarray
+#  - name: memalign
+#  - name: valloc
+# Each of those glibc headers also has related functions the other lacks.
+# Only the common subset is mentioned here for future consideration.

libc/include/stdlib-malloc.yaml

@@ -2,6 +2,8 @@ header: stdlib.h
 header_template: stdlib.h.def
 standards:
   - stdc
+merge_yaml_files:
+  - stdlib-malloc.yaml
 macros: []
 types:
   - type_name: __atexithandler_t
@@ -34,13 +36,6 @@ functions:
     return_type: int
     arguments:
       - type: int
-  - name: aligned_alloc
-    standards:
-      - stdc
-    return_type: void *
-    arguments:
-      - type: size_t
-      - type: size_t
   - name: at_quick_exit
     standards:
       - stdc
@@ -87,13 +82,6 @@ functions:
       - type: size_t
       - type: size_t
       - type: __bsearchcompare_t
-  - name: calloc
-    standards:
-      - stdc
-    return_type: void *
-    arguments:
-      - type: size_t
-      - type: size_t
   - name: div
     standards:
       - stdc
@@ -107,12 +95,6 @@ functions:
     return_type: _Noreturn void
     arguments:
       - type: int
-  - name: free
-    standards:
-      - stdc
-    return_type: void
-    arguments:
-      - type: void *
   - name: getenv
     standards:
       - stdc
@@ -145,12 +127,6 @@ functions:
     arguments:
       - type: long long
       - type: long long
-  - name: malloc
-    standards:
-      - stdc
-    return_type: void *
-    arguments:
-      - type: size_t
   - name: qsort
     standards:
       - stdc
@@ -182,13 +158,6 @@ functions:
     return_type: int
     arguments:
       - type: void
-  - name: realloc
-    standards:
-      - stdc
-    return_type: void *
-    arguments:
-      - type: void *
-      - type: size_t
   - name: srand
     standards:
       - stdc