[libc] Fix the wrapper headers for 'toupper' and 'tolower'

Summary:
The GNU headers like to reassign this function to a new function which
the optimizer will pick up unless compiling with `O0`. This uses an
external LUT which we don't have and fails to link. This patch makes
sure that the GPU portion does not include these extra definitions and
we only use the ones we support. It's hacky, but it's the only way to
disable it.

clang/lib/Headers/llvm_libc_wrappers/ctype.h

@@ -13,8 +13,19 @@
 #error "This file is for GPU offloading compilation only"
 #endif
 
+// The GNU headers like to define 'toupper' and 'tolower' redundantly. This is
+// necessary to prevent it from doing that and remapping our implementation.
+#if (defined(__NVPTX__) || defined(__AMDGPU__)) && defined(__GLIBC__)
+#pragma push_macro("__USE_EXTERN_INLINES")
+#undef __USE_EXTERN_INLINES
+#endif
+
 #include_next <ctype.h>
 
+#if (defined(__NVPTX__) || defined(__AMDGPU__)) && defined(__GLIBC__)
+#pragma pop_macro("__USE_EXTERN_INLINES")
+#endif
+
 #if __has_include(<llvm-libc-decls/ctype.h>)
 
 #if defined(__HIP__) || defined(__CUDA__)