pkgbase: remove incorrect clang shlib requires

The FreeBSD-clang package contains a 32-bit shared object at
/usr/lib/clang/19/lib/freebsd/libclang_rt.asan-i386.so

This is expected, since clang uses this object when compiling for i386
targets with asan enabled.

What is not expected is that the FreeBSD-clang package currently depends
on 32-bit libc packages due to pkg's shared library analysis, making it
impossible to install pkgbase on x86_64 without any lib32 packages.

This commit leverages a new pkg feature implemented in [1], but could
be landed before a pkg version including that feature is released
without any ill effects. Unknown keys in package manifests are ignored.

[1]: https://github.com/freebsd/pkg/pull/2594

Reviewed by: ivy
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D54792

(cherry picked from commit 2018ae4e3b6a2d7c147933cb36642f6a54830907)