arc: add support of ARC HS38 core

Synopsys has recently announced its new ARC HS38 core that is capable of
running Linux -
http://www.synopsys.com/dw/ipdir.php?ds=arc-hs38-processor



ARC HS38 is based on ARCv2 ISA and requires special settings of gcc and
libc.

Also in case of HS38 atomic extensions (LLOCK/SCOND instructions) are
built-in by default, so enabling atomic extensions in Buildroot as well.

This commit adds support of the core in buildroot.

[Peter: string type, so must be in quotes as noted by Yann]
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Anton Kolesov <anton.kolesov@synopsys.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>