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/cc-wrapper/default.nix

@@ -232,7 +232,7 @@ stdenv.mkDerivation {
       # compile, because it uses "#include_next <limits.h>" to find the
       # limits.h file in ../includes-fixed. To remedy the problem,
       # another -idirafter is necessary to add that directory again.
-      echo "-B${libc_lib}/lib/ -idirafter ${libc_dev}/include ${optionalString isGNU "-idirafter ${cc}/lib/gcc/*/*/include-fixed"}" > $out/nix-support/libc-cflags
+      echo "-B${libc_lib}${libc.libdir or "/lib/"} -idirafter ${libc_dev}${libc.incdir or "/include"} ${optionalString isGNU "-idirafter ${cc}/lib/gcc/*/*/include-fixed"}" > $out/nix-support/libc-cflags
 
       echo "${libc_lib}" > $out/nix-support/orig-libc
       echo "${libc_dev}" > $out/nix-support/orig-libc-dev
@@ -284,6 +284,20 @@ stdenv.mkDerivation {
       hardening_unsupported_flags+=" stackprotector"
     ''
 
+    + optionalString targetPlatform.isAvr ''
+      hardening_unsupported_flags+=" stackprotector pic"
+    ''
+
+    + optionalString (targetPlatform.libc == "newlib") ''
+      hardening_unsupported_flags+=" stackprotector fortify pie pic"
+    ''
+
+    + 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 "-B${getLib libc}/avr/lib/$isa" >> $out/nix-support/libc-cflags
+      done
+    ''
+
     + ''
       substituteAll ${./add-flags.sh} $out/nix-support/add-flags.sh
       substituteAll ${./add-hardening.sh} $out/nix-support/add-hardening.sh