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
Cleanup up ARM *frame structures...

 - Eliminate unused irqframe
 - Eliminate unused saframe
 - Instead of splitting r4-sp storage between the stack and switchframe,
   just put all the registers in switchframe and eliminate the un_32 struct.

Submitted by:	Svatopluk Kraus <onwahe@gmail.com>,
		Michal Meloun <meloun@miracle.cz>