ARC: build: Try to guess CROSS_COMPILE with cc-cross-prefix

For a long time we used to hard-code CROSS_COMPILE prefix
for ARC until it started to cause problems, so we decided to
solely rely on CROSS_COMPILE externally set by a user:
commit 40660f1f ("ARC: build: Don't set CROSS_COMPILE in arch's Makefile").

While it works perfectly fine for build-systems where the prefix
gets defined anyways for us human beings it's quite an annoying
requirement especially given most of time the same one prefix
"arc-linux-" is all what we need.

It looks like finally we're getting the best of both worlds:
 1. W/o cross-toolchain we still may install headers, build .dtb etc
 2. W/ cross-toolchain get the kerne built with only ARCH=arc

Inspired by [1] & [2].

[1] http://lists.infradead.org/pipermail/linux-snps-arc/2019-May/005788.html
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=fc2b47b55f17



A side note: even though "cc-cross-prefix" does its job it pollutes
console with output of "which" for all the prefixes it didn't manage to find
a matching cross-compiler for like that:
| # ARCH=arc make defconfig
| which: no arceb-linux-gcc in (~/.local/bin:~/bin:/usr/bin:/usr/sbin)
| *** Default configuration is based on 'nsim_hs_defconfig'

Suggested-by: Vineet Gupta <vgupta@synopsys.com>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>