[libomptarget][amdgpu] don't declare Elf_Note on FreeBSD

[dim]

Summary

On FreeBSD, the system <libelf.h> already declares struct Elf_Note
indirectly (via <sys/elf_common.h>). This results in compile errors
when building the libomptarget amdgpu plugin. Avoid redeclaring `struct
Elf_Note` on FreeBSD to fix the errors.

[JonChesterfield]

Thank you for the patch! I haven't been compiling this on a bsd.

Adding it to the list of reasons to replace libelf with the llvm elf headers within this plugin

openmp/libomptarget/plugins/amdgpu/impl/system.cpp

return hsa_executable_iterate_symbols(executable, L,
                                         static_cast<void *>(&cb));
 }
 } // namespace hsa
 
 typedef unsigned char *address;
 /*
  * Note descriptors.
  */
+// FreeBSD already declares Elf_Note (indirectly via <libelf.h>)
+#if !defined(__FreeBSD__)
 typedef struct {
   uint32_t n_namesz; /* Length of note's name. */
   uint32_t n_descsz; /* Length of note's value. */
   uint32_t n_type;   /* Type of note. */
   // then name
   // then padding, optional
   // then desc, at 4 byte alignment (not 8, despite being elf64)
 } Elf_Note;
+#endif
 
 // The following include file and following structs/enums
 // have been replicated on a per-use basis below. For example,
 // llvm::AMDGPU::HSAMD::Kernel::Metadata has several fields,
 // but we may care only about kernargSegmentSize_ for now, so
 // we just include that field in our KernelMD implementation. We
 // chose this approach to replicate in order to avoid forcing
 // a dependency on LLVM_INCLUDE_DIR just to compile the runtime.