math/trlan: fix build for base system WITH_LLVM_BINUTILS

Use strip --strip-unneeded instead of strip -x. strip -x strips all
non-global symbols, but there are some non-global symbols referenced by
relocations. ELF Tool Chain strip and GNU strip silently retain those
symbols, but LLVM strip is more strict and reports an error.

PR: 270958
Reviewed by: jrm
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39754