[compiler-rt] Add missing #else clause to fix the build on NetBSD.

[fcambus]

Summary

An #elif SANITIZER_SOLARIS clause was removed in D120048, but it also removed the #else clause for the error fallback, causing the build to fail on NetBSD.

[MaskRay]

Thanks!

compiler-rt/lib/sanitizer_common/sanitizer_linux_libcdep.cpp

if (tcb != 0) {
     // ld.elf_so hardcodes the index 1.
     dl_iterate_phdr(GetSizeFromHdr, size);
 
     if (*size != 0) {
       // The block has been found and tcb_dtv[1] contains the base address
       *addr = (uptr)tcb->tcb_dtv[1];
     }
   }
+#else
 #error "Unknown OS"
 #endif
 }
 #endif
 
 #if !SANITIZER_GO
 uptr GetTlsSize() {
 #if SANITIZER_FREEBSD || SANITIZER_LINUX || SANITIZER_NETBSD || \