[libc] Link the SCUDO integration tests to a special entrypoint collection.

[sivachandra]

Summary

We were previously linking to libllvmlibc.a. But, with libllvmlibc.a now
including functions which depend on the loader, we will have to use the
LLVM libc loader as well. To avoid this, we will link to a special
library which is just a collection of SCUDO allocator entrypoints.

[michaelrj]

LGTM, although I should look into adding an entrypoint for aligned_alloc

libc/test/integration/scudo/CMakeLists.txt

 if(NOT LLVM_LIBC_INCLUDE_SCUDO)
   return()
 endif()
 
+# We use a special library consisting of only the SCUDO allocator
+# functions to link to the integration tests below. We could instead
+# link to libllvmlibc.a directly, but since libllvmlibc.a contains
+# functions which depend on the LLVM libc loader, the integration
+# test will have to link to the LLVM libc loader. LLVM libc's loader
+# is not complete enough to allow this. It is also desireable to
+# keep the dependencies as minimal as possible.
+add_entrypoint_library(
+  libc_for_scudo_integration_test
+  DEPENDS
+    libc.src.stdlib.malloc
+    libc.src.stdlib.calloc
+    libc.src.stdlib.realloc
+    libc.src.stdlib.free
+)
+
 add_executable(
   libc-scudo-integration-test
   integration_test.cpp
 )
 
 target_link_options(
   libc-scudo-integration-test
   PRIVATE
-  # TODO: Uncomment "-nolibc" once llvm-libc is complete enough.
-  # "-nolibc"
   -pthreads
 )
 
 target_link_libraries(libc-scudo-integration-test
   PRIVATE
-  llvmlibc
+  libc_for_scudo_integration_test
 )
 
 add_executable(
   libc-gwp-asan-uaf-should-crash
   gwp_asan_should_crash.cpp
 )
 
 target_link_options(
   libc-gwp-asan-uaf-should-crash
   PRIVATE
-  # TODO: Uncomment "-nolibc" once llvm-libc is complete enough.
-  # "-nolibc"
   -pthreads
 )
 
 target_link_libraries(libc-gwp-asan-uaf-should-crash
   PRIVATE
-  llvmlibc
+  libc_for_scudo_integration_test
 )
 
 add_custom_command(TARGET libc-scudo-integration-test
                    POST_BUILD
                    COMMAND $<TARGET_FILE:libc-scudo-integration-test>
                    COMMENT "Run the test after it is built." 
                    VERBATIM)