AVR support

This is mainly motivated due to size issues when using LLVM when AVR is targeted.

I have partial support for it in my forks (gcc, gccjit.rs, rustc_codegen_gcc, but there are a few features needed and some issues to be addressed.

Features needed:

• Setting mode flags (-m) in libgccjit.
  • Done in fork, yet to be sent and more switches could be added.
• Fixed-point types.
  • Done in fork, yet to be sent.
• Specify that sysroot should only depend on core and alloc for AVR.

Issues to be fixed:

• Setting an MCU to an specific one, such as atmega328p, fails with an error message.
  • Done in fork by commenting lines, but a better solution is needed.
• $GCC_EXEC_PATH needs to be set for it to pick device-specs at the correct location (lib/gcc/avr/<gcc version>).
  • Done in fork but is a bit of a kludge.
• Doubles have a width of 32 bits by default.
  • Done in fork, yet to be sent.
• Volatiles loads and stores are not honored.
  • As seen in #167.
  • Related to #166.
  • Fix sent to GCC jit ML.
• In some binaries sizes are bigger than LLVM as of now due to panicking code not being optimized.
  • Should be tracked in a separate issue.
• Hex dumps using avr-objcopy -O ihex are way bigger than C.
  • Could be related to the previous issue.
• Garbage is transmitted when writing to a serial device in a dev build, release builds are not affected.
  • Should be tracked in a separate issue.

No project files or tests are named. Start by reviewing the linked AVR-support forks and the libgccjit and rustc_codegen_gcc changes, then narrow the checklist to one unchecked item, considering the related issues #166 and #167. Done means one focused AVR feature or fix is implemented and its behavior is validated.