[libc] Use the NVIDIA device allocator for GPU malloc

Summary:
This is a blocker on another patch in the OpenMP runtime. The problem is
that NVIDIA truly doesn't handle RPC-based allocations very well. It
cannot reliably update the MMU while a kernel is running and it will
usually deadlock if called from a separate thread due to internal use of
TLS.

This patch just removes the definition of `malloc` and `free` for NVPTX.
The result here is that they will be undefined, which is the cue for the
`nvlink` linker to define them for us. So, as far as `libc` is concerned
it still implements malloc.

Author: jhuber6

libc/src/stdlib/gpu/free.cpp

@@ -14,6 +14,10 @@
 
 namespace LIBC_NAMESPACE_DECL {
 
+// FIXME: For now we just default to the NVIDIA device allocator which is
+// always available on NVPTX targets. This will be implemented fully later.
+#ifndef LIBC_TARGET_ARCH_IS_NVPTX
 LLVM_LIBC_FUNCTION(void, free, (void *ptr)) { gpu::deallocate(ptr); }
+#endif
 
 } // namespace LIBC_NAMESPACE_DECL

libc/src/stdlib/gpu/malloc.cpp

@@ -14,8 +14,12 @@
 
 namespace LIBC_NAMESPACE_DECL {
 
+// FIXME: For now we just default to the NVIDIA device allocator which is
+// always available on NVPTX targets. This will be implemented fully later.
+#ifndef LIBC_TARGET_ARCH_IS_NVPTX
 LLVM_LIBC_FUNCTION(void *, malloc, (size_t size)) {
   return gpu::allocate(size);
 }
+#endif
 
 } // namespace LIBC_NAMESPACE_DECL

libc/test/src/stdlib/CMakeLists.txt

@@ -420,7 +420,8 @@ if(LLVM_LIBC_FULL_BUILD)
   )
 
   # Only baremetal and GPU has an in-tree 'malloc' implementation.
-  if(LIBC_TARGET_OS_IS_BAREMETAL OR LIBC_TARGET_OS_IS_GPU)
+  if((LIBC_TARGET_OS_IS_BAREMETAL OR LIBC_TARGET_OS_IS_GPU) AND
+      NOT LIBC_TARGET_ARCHITECTURE_IS_NVPTX)
     add_libc_test(
       malloc_test
       HERMETIC_TEST_ONLY