Remove historical GNUC test

The requirement is for a GCC-compatible compiler and not necessarily
GCC itself. However, we currently expect any compiler used for building
the whole of FreeBSD to be GCC-compatible and many things will break if
not; there's no longer a need to have an explicit test for this in rtld.

Reviewed by:	imp, kib
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D2422

libexec/rtld-elf/debug.h

@@ -32,10 +32,6 @@
 #ifndef DEBUG_H
 #define DEBUG_H 1
 
-#ifndef __GNUC__
-#error "This file must be compiled with GCC"
-#endif
-
 #include <sys/cdefs.h>
 
 #include <string.h>

libexec/rtld-elf/rtld.c

@@ -34,10 +34,6 @@
  * John Polstra <jdp@polstra.com>.
  */
 
-#ifndef __GNUC__
-#error "GCC is needed to compile this file"
-#endif
-
 #include <sys/param.h>
 #include <sys/mount.h>
 #include <sys/mman.h>