[avr-libc-dev] [Docs] on compiler / linker flags

[flo . kerle]

Hello,

TLDR: I forgot to set this flag for the linker also. (No note at the first 
paragraph, and no note under "selected linker options")
 IMHO there *should* be a mention in the first paragraph, that this flag is OFC 
needed for the linker also. At least it should be the first flag for "selected 
linker options"
Background, to make a mock of myself:
Recently I've adapted mv CMakeLists.txt from ARM to AVR targets. As blind as I 
was, I did read the docs on "C compiler flags", and encountered `-mmcu=`.
Mentioned for the compiler, not the linker. Added the flag, did not turn some 
grey cells on to ask "what does the linker do, and what does it need for that 
you idiot?"

Very weired issues popped up. Only Emil on the #avrs IRC channel shed light 
onto my blindness, and I got that linker doing what it was supposed to.
To save others that hassle, I'd like to make the docs more foolproof.
Best regards,
Florian