[libc] Fill out generated malloc.h and related stdlib.h extensions (#127293)

This updates the generated stdlib.h and malloc.h headers to
include the subsets of extenion functions declared by glibc that
are also supported by Scudo and that use only simple types.
Scudo's extensions not declared by glibc are omitted.  glibc's
extensions not implemented by Scudo are omitted.  The mallinfo
and mallinfo2 functions are omitted (at least for now) since they
need struct definitions for their return types.

libc/include/malloc.yaml

@@ -9,6 +9,12 @@ macros:
   - macro_name: M_PURGE_ALL
     macro_header: malloc-macros.h
 functions:
+  - name: malloc_usable_size
+    standards:
+      - gnu
+    return_type: size_t
+    arguments:
+      - type: void *
   - name: mallopt
     standards:
       - gnu
@@ -16,3 +22,10 @@ functions:
     arguments:
       - type: int
       - type: int
+  - name: pvalloc
+    standards:
+      - bsd
+      - gnu
+    return_type: void *
+    arguments:
+      - type: size_t

libc/include/stdlib-malloc.yaml

@@ -1,4 +1,10 @@
 # This file has declarations that appear both in <stdlib.h> and in <malloc.h>.
+# These include the subset of GNU extensions that Scudo supports.
+#
+# Note: glibc's <stdlib.h> and <malloc.h> both also have `reallocarray`,
+# which Scudo does not support and is omitted here.  (Each of those glibc
+# headers also has related functions the other lacks, but those should be
+# covered separately in stdlib.yaml and malloc.yaml instead.)
 
 functions:
   - name: aligned_alloc
@@ -27,6 +33,13 @@ functions:
     return_type: void *
     arguments:
       - type: size_t
+  - name: memalign
+    standards:
+      - gnu
+    return_type: void *
+    arguments:
+      - type: size_t
+      - type: size_t
   - name: realloc
     standards:
       - stdc
@@ -34,11 +47,10 @@ functions:
     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.
+  - name: valloc
+    standards:
+      - bsd
+      - gnu
+    return_type: void *
+    arguments:
+      - type: size_t

libc/include/stdlib.yaml

@@ -127,6 +127,14 @@ functions:
     arguments:
       - type: long long
       - type: long long
+  - name: posix_memalign
+    standards:
+      - posix
+    return_type: int
+    arguments:
+      - type: void **
+      - type: size_t
+      - type: size_t
   - name: qsort
     standards:
       - stdc