vdso: Build without debug info

The debug info is not very useful and embeds build paths because
elf-vdso.so.o is built without CFLAGS, breaking reproducibility of
elf-vdso.so.o and thus the kernel. Just stop passing debug flags to
amd64_vdso.sh.

Reviewed by: kib
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52129

(cherry picked from commit b7e0373acb1d022e9e5acb5be9727def5f941194)