Remove CROSS_COMPILE default from arch/*/config.mk

In order to support the compiler providing information used within
Kconfig itself we cannot have the compiler be determined by
arch/*/config.mk as we will not be able to evaluate that yet.  Given
that most documentation tells people to specify CROSS_COMPILE, remove
these references.

Cc: Huan Wang <alison.wang@nxp.com>
Cc: Angelo Dureghello <angelo@sysam.it>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Rick Chen <rick@andestech.com>
Cc: Thomas Chou <thomas@wytron.com.tw>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Michal Simek <michal.simek@xilinx.com>