devel/boehm-gc: update to 7.6.4

• Regenerate patch to make portlint happy
• Bump SHLIBS version in pkg-plist

Changelog: https://github.com/ivmai/bdwgc/releases/tag/v7.6.4

PR: 225560
Submitted by: Yasuhiro KIMURA <yasu@utahime.org>
Reviewed by: cpm
Exp-run by: antoine