devel/avr-gcc: Fix build with libc++ 17

gcc/system.h includes safe-ctype.h which redefines ctype macros such
as toupper, tolower, etc to "poison" them.

However, it should only include the safe-ctype.h header *after* any
C++ headers, such as <list>, <map>, <string>, etc, otherwise these
might transitively include internal ctype headers (such as with libc++
17), causing compilation errors.

PR: 274038
Reported by: dim