[compiler-rt] Define ElfW() macro if it's not defined

Summary:
The `ElfW()` macro is not provided by `<link.h>` on some
systems (e.g., FreeBSD). On these systems the data structures are
just called `Elf_XXX`. Define `ElfW()` locally.

(This fix is taken from [libunwind](9b05596eff/libunwind/src/AddressSpace.hpp (L144-L157)).)

Reviewers: compnerd

Differential revision: https://reviews.llvm.org/D75907

compiler-rt/lib/sanitizer_common/sanitizer_linux_libcdep.cpp

@@ -35,6 +35,10 @@
 #include <sys/resource.h>
 #include <syslog.h>
 
+#if !defined(ElfW)
+#define ElfW(type) Elf_##type
+#endif
+
 #if SANITIZER_FREEBSD
 #include <pthread_np.h>
 #include <osreldate.h>