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