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

lib/systems/examples.nix

@@ -99,9 +99,23 @@ rec {
   riscv64 = riscv "64";
   riscv32 = riscv "32";
 
-  arduino-uno = {
+  avr = {
     config = "avr";
-    platform = { name = "avr5"; };
+  };
+
+  arm-embedded = {
+    config = "arm-none-eabi";
+    libc = "newlib";
+  };
+
+  aarch64-embedded = {
+    config = "aarch64-none-elf";
+    libc = "newlib";
+  };
+
+  ppc-embedded = {
+    config = "powerpc-none-eabi";
+    libc = "newlib";
   };
 
   #