math/z3: Update 4.15.4.0 => 4.15.8.0

Changelogs:

• https://github.com/Z3Prover/z3/blob/master/RELEASE_NOTES.md#version-4158
• https://github.com/Z3Prover/z3/compare/z3-4.15.4...z3-4.15.5
• https://github.com/Z3Prover/z3/compare/z3-4.15.5...z3-4.15.6
• https://github.com/Z3Prover/z3/compare/z3-4.15.6...z3-4.15.7
• https://github.com/Z3Prover/z3/compare/z3-4.15.7...z3-4.15.8

Approved by: arrowd@ (maintainer)
Approved by: db@, yuri@ (Mentors, implicit)
Differential Revision: https://reviews.freebsd.org/D55324