[Libomptarget] Make the references to 'malloc' and 'free' weak. (#69356)

Summary:
We use `malloc` internally in the DeviceRTL to handle data
globalization. If this is undefined it will map to the Nvidia
implementation of `malloc` for NVPTX and return `nullptr` for AMDGPU.
This is somewhat problematic, because when using this as a shared
library it causes us to always extract the GPU libc implementation,
which uses RPC and thus requires an RPC server. Making this `weak`
allows us to implement this internally without worrying about binding to
the GPU `libc` implementation.

Author: jhuber6

openmp/libomptarget/DeviceRTL/src/State.cpp

@@ -46,8 +46,8 @@ namespace {
 ///{
 
 extern "C" {
-__attribute__((leaf)) void *malloc(uint64_t Size);
-__attribute__((leaf)) void free(void *Ptr);
+[[gnu::weak, gnu::leaf]] void *malloc(uint64_t Size);
+[[gnu::weak, gnu::leaf]] void free(void *Ptr);
 }
 
 ///}

openmp/libomptarget/DeviceRTL/src/exports

@@ -11,6 +11,8 @@ _ZN4ompx*
 
 IsSPMDMode
 
+malloc
+free
 memcmp
 printf
 __assert_fail