lang/gcc>=11: Warn about LTO_BOOTSTRAP

Warn users about the amount of ram and time needed to build GCC with
LTO_BOOTSTRP enabled.

PR: 264949
Reported by: Tomoaki AOKI <junchoon@dec.sakura.ne.jp> and others
Reviewed by: gerald
Differential Revision: https://reviews.freebsd.org/D35688