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/build-support/bintools-wrapper/default.nix

@@ -210,7 +210,7 @@ stdenv.mkDerivation {
       ## General libc support
       ##
 
-      echo "-L${libc_lib}/lib" > $out/nix-support/libc-ldflags
+      echo "-L${libc_lib}${libc.libdir or "/lib"}" > $out/nix-support/libc-ldflags
 
       echo "${libc_lib}" > $out/nix-support/orig-libc
       echo "${libc_dev}" > $out/nix-support/orig-libc-dev
@@ -293,6 +293,16 @@ stdenv.mkDerivation {
       hardening_unsupported_flags+=" pic"
     ''
 
+    + optionalString targetPlatform.isAvr ''
+      hardening_unsupported_flags+=" relro bindnow"
+    ''
+
+    + optionalString (libc != null && targetPlatform.isAvr) ''
+      for isa in avr5 avr3 avr4 avr6 avr25 avr31 avr35 avr51 avrxmega2 avrxmega4 avrxmega5 avrxmega6 avrxmega7 tiny-stack; do
+        echo "-L${getLib libc}/avr/lib/$isa" >> $out/nix-support/libc-cflags
+      done
+    ''
+
     + ''
       set +u
       substituteAll ${./add-flags.sh} $out/nix-support/add-flags.sh