Replace using of objdump with elfdump

In-tree objdump is too old to dump new ELF headers. But for example if we
use: make CROSS_TOOLCHAIN=riscv64-gcc TARGET_ARCH=riscv64 and do not specify
CROSS_BINUTILS_PREFIX in env, embed_mfs.sh cannot find the correct objdump.
This patch just replaces using of objdump with elfdump to collect needed
information.

Later we may also put an ELFDUMP in CROSSENV and use it in embed_mfs.sh .

Reviewed by: emaste, br
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D9062