[libc] Enable the GPU build to build the UnitTest library

[jhuber6]

Summary

The previous patch in D149216 allows us to use the internal <stdlib.h>
include for the GPU build. However, we currently don't provide the
memory functions so the header wasn't resolving them. This patch adds
these as entrypoints. They don't cause any entrypoints to be emitted
because they are not implemented, but they provide it in the header so
that we can rely on the test's implementation of them.

Depends on D149216

libc/config/gpu/entrypoints.txt

set(TARGET_LIBC_ENTRYPOINTS
     libc.src.string.strspn
     libc.src.string.strstr
     libc.src.string.strtok
     libc.src.string.strtok_r
 
     # stdlib.h entrypoints
     libc.src.stdlib.atoi
 
+    # Only implemented in the test suite
+    libc.src.stdlib.malloc
+    libc.src.stdlib.realloc
+    libc.src.stdlib.free
+
     # errno.h entrypoints
     libc.src.errno.errno
 )
 
 set(TARGET_LLVMLIBC_ENTRYPOINTS
   ${TARGET_LIBC_ENTRYPOINTS}
 )
 

libc/test/CMakeLists.txt

 add_custom_target(check-libc)
 add_custom_target(libc-unit-tests)
 add_custom_target(libc-hermetic-tests)
 add_dependencies(check-libc libc-unit-tests libc-hermetic-tests)
 
 add_custom_target(exhaustive-check-libc)
 add_custom_target(libc-long-running-tests)
 
-if(NOT LIBC_TARGET_ARCHITECTURE_IS_GPU)
-  add_subdirectory(UnitTest)
-endif()
 
 add_header_library(
   errno_setter_matcher
   HDRS
     ErrnoSetterMatcher.h
   DEPENDS
     libc.src.errno.errno
 )