Reduce build times

U-Boot Makefiles contain a number of tests for compiler features etc.
which so far are executed again and again.  On some architectures
(especially ARM) this results in a large number of calls to gcc.

This patch makes sure to run such tests only once, thus largely
reducing the number of "execve" system calls.

Example: number of "execve" system calls for building the "P2020DS"
(Power Architecture) and "qong" (ARM) boards, measured as:
	-> strace -f -e trace=execve -o /tmp/foo ./MAKEALL <board>
	-> grep execve /tmp/foo | wc -l

	Before: After:	Reduction:
==================================
P2020DS 20555	15205	-26%
qong	31692	14490	-54%

As a result, built times are significantly reduced, typically by
30...50%.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Andy Fleming <afleming@gmail.com>
Cc: Kumar Gala <galak@kernel.crashing.org>
Cc: Albert Aribaud <albert.aribaud@free.fr>
cc: Graeme Russ <graeme.russ@gmail.com>
cc: Mike Frysinger <vapier@gentoo.org>
Tested-by: Graeme Russ <graeme.russ@gmail.com>
Tested-by: Matthias Weisser <weisserm@arcor.de>
Tested-by: Sanjeev Premi <premi@ti.com>
Tested-by: Simon Glass <sjg@chromium.org>
Tested-by: Macpaul Lin <macpaul@gmail.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>