Build gperf before gcc
The ARM image builds build the in-tree gcc in order to build u-boot
and gperf is needed to build gcc, but is no longer installed on archs
that use clang.
Invoking the make targets as separate steps is done to work around
a build failure which is not yet fully understood.
Reviewed by: gjb, imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D1317