ARC: switch to arc-linux- CROSS_COMPILE prefix across all configs

When building kernel with buildroot built toolchain, CROSS_COMPILE
currently needs adjustment even if minor. This is because the defconfigs
prefer "arc-linux-uclibc-" prefix from hand built (non buildroot) toolchain
while buildroot provides "arc-buildroot-linux-uclibc-"

To avoid this use the common "arc-linux-" prefix which is provided by
buildroot and has also been in hand built tools for quite some time.

Signed-off-by: sujayraaj <sujayraaj@gmail.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
[vgupta: updated changelog]