[libc] Breakup freelist_malloc into separate files (#119806)

This better matches the structure we use for the rest of libc.

libc/fuzzing/__support/CMakeLists.txt

@@ -24,10 +24,21 @@ add_libc_fuzzer(
     -D__LIBC_EXPLICIT_SIMD_OPT
 ) 
 
-add_libc_fuzzer(
-  freelist_heap_fuzz
-  SRCS
-    freelist_heap_fuzz.cpp
-  DEPENDS
-    libc.src.__support.freelist_heap
-)
+if(LLVM_LIBC_FULL_BUILD)
+  add_libc_fuzzer(
+    freelist_heap_fuzz
+    SRCS
+      fake_heap.s
+      freelist_heap_fuzz.cpp
+    DEPENDS
+      libc.src.__support.freelist_heap
+  )
+  # TODO(#119995): Remove this once sccache on Windows no longer requires
+  # the use of -DCMAKE_MSVC_DEBUG_INFORMATION_FORMAT=Embedded.
+  get_fq_target_name(freelist_heap_fuzz freelist_heap_fuzz_target_name)
+  set_target_properties(
+    ${freelist_heap_fuzz_target_name}
+    PROPERTIES
+      MSVC_DEBUG_INFORMATION_FORMAT ""
+  )
+endif()

libc/fuzzing/__support/fake_heap.s

@@ -0,0 +1,15 @@
+//===-- Test fake definition for heap symbols -----------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+.globl _end, __llvm_libc_heap_limit
+
+.bss
+_end:
+.fill 1024
+__llvm_libc_heap_limit:
+