link.h: add glibc-compatible ElfW(type) macro

Portable ELF code (Mesa util/build_id.c, unwinders) uses ElfW(Nhdr) etc. and
expects <link.h> to define ElfW -> Elf64_type (LP64) / Elf32_type. Without it
those TUs fall back to an undefined Elf_##type and fail to compile.

include/link.h

@@ -12,6 +12,20 @@
 #include <elf.h>
 #include <stddef.h>
 
+/*
+ * glibc-compatible `ElfW(type)` macro: expands to the native-word-size ELF type
+ * (Elf64_type on LP64, Elf32_type otherwise). Portable ELF code (Mesa's
+ * util/build_id.c, unwinders) uses `ElfW(Nhdr)` etc. and expects <link.h> to
+ * provide this.
+ */
+#ifndef ElfW
+# if defined(__LP64__) || defined(_LP64) || (__SIZEOF_POINTER__ == 8)
+#  define ElfW(type) Elf64_##type
+# else
+#  define ElfW(type) Elf32_##type
+# endif
+#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif