Backport two fixes from lang/gcc6:

[1] Make sure what we install is stripped (i.e., debug info is removed).

(For more background see revisions 454177 and 454422.)

[2] Add a patch that we pulled into gcc6-devel via upstream a week ago

that addresses a real-world issue around threading and unwinding as
files/patch-freebsd-unwind.h .

Bump PORTREVISION since [2] is a functional change and [1] changes the
package.

Reported by: Ports QA Framework, miwi, sobomax [1]
Discussed with: tijl, miwi [1]
Tested by: sobomax [1]
Differential Revision: https://reviews.freebsd.org/D10357 [1]