Backport the change to ensure what we install is stripped (i.e., debug
info is removed) from lang/gcc7 to lang/gcc47, lang/gcc48 and lang/gcc49.

(For more background see revisions 454177 and 454422.)

Reported by: Ports QA Framework, miwi, sobomax
Discussed with: tijl, miwi
Differential Revision: https://reviews.freebsd.org/D10357