arm64: Don't rely on host readelf for u-boot booti image generation

readelf is not a bootstrap tool and so cannot be relied upon to exist.
On macOS there is no system readelf, and even on Linux or FreeBSD where
it does exist, BUILD_WITH_STRICT_TMPPATH builds won't be able to use it.
Instead of making it a bootstrap tool, just use nm as that suffices and
already is a bootstrap tool.

Fixes: 28482babd08a ("arm64: Use new arm_kernel_boothdr script for generating booti images.")
Reviewed by: emaste, mmel
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D32734