devel/elfutils: Unbreak gcc8 build

Drop one new gcc8 warning, and fix another in the FreeBSD glibc-
compatibility shim patch.

Re-enable gcc8 option (USE_GCC= yes).

Bump PORTREVISION due to the patch change (no functional change).

PR: 232932
Reported by: gerald, kib
Reviewed by: bdrewery
Approved by: bdrewery
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D17882