ARC: Add support for ARC HS38 with Quad MAC & FPU

ARC: Add support for ARC HS38 with Quad MAC & FPU

We used to build everything for pretty much baseline ARC HS capable
of runnig Linux kernel, which was ARC HS38/48 with MMU and caches.

But there's a fully featured ARC HS with additional support for
 - Dual & quad integer multiply and MAC operations
 - Double-precision floating-point unit

It corresponds to the following ARC HS templates in ARChitect: hs38_slc_full.

In fact existing HSDK board uses exactly this configuration in its SoC
and this is recommended configuration for Linux use-cases.

To make life simpler we have corresponding "-mcpu" and "--with-cpu"
options in ARC GCC port so we're going to use it and get binaries
built accordingly optimized.

And while at it added help message so users may better understand
what they are dealing with.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>