devel/pcre2: Correctly detect OpenPOWER 64-bit ELFv2 ABI

PR: 239408
Submitted by: bdragon
Reported by: bdragon
Approved by: krion (maintainer), linimon (mentor)
Obtained from: https://bugs.exim.org/show_bug.cgi?id=2353
Differential Revision: https://reviews.freebsd.org/D21063