Bump Linux branch to arc-3.15

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>
Use RHEL Mingw by default

Ubuntu defines MinGW toolchain as i686-w64-mingw32, while RHEL as
i686-pc-mingw32. This patch makes RHEL variant default in the build-elf32.sh
patch for Windows toolchain.

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>
Rename EM to arcem and HS to archs in build scripts

GCC option has been renamed to arcem and archs, so build scripts should
match it.

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>
Add a Makefile to create prebuilt packages automatically

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>
Update readme to current state of tools

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>