[AVR] Automatically link CRT and libgcc from the system avr-gcc

Summary:
This patch modifies the AVR toolchain so that if avr-gcc and avr-libc
are detected during compilation, the CRT, libgcc, libm, and libc anre
linked.

This matches avr-gcc's default behaviour, and the expected behaviour of
all C compilers - including the C runtime.

avr-gcc also needs a -mmcu specified in order to link runtime libraries.

The difference betwen this patch and avr-gcc is that this patch will
warn users whenever they compile without a runtime, as opposed to GCC,
which silently trims the runtime libs from the linker arguments when no
-mmcu is specified.

Reviewers: aaron.ballman, kparzysz, asb, hfinkel, brucehoult, TimNN

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D54334

llvm-svn: 361116