gcc: support avr

- respect libc’s incdir and libdir
- make non-unix systems single threaded
- set LIMITS_H_TEST to false for avr
- misc updates to support new libc’s
- use multilib with avr

For threads we want to use:
- posix on unix systems
- win32 on windows
- single on everything else

For avr:
- add library directories for avrlibc
- to disable relro and bind
- avr5 should have precedence over avr3 - otherwise gcc uses the wrong one

pkgs/development/misc/avr/libc/default.nix

@@ -14,9 +14,13 @@ stdenv.mkDerivation {
   nativeBuildInputs = [ automake autoconf ];
 
   # Make sure we don't strip the libraries in lib/gcc/avr.
-  stripDebugList= "bin";
+  stripDebugList = "bin";
   dontPatchELF = true;
 
+  passthru = {
+    incdir = "/avr/include";
+  };
+
   meta = with stdenv.lib; {
     description = "a C runtime library for AVR microcontrollers";
     homepage = http://savannah.nongnu.org/projects/avr-libc/;