gcc: arc: Download from github instead of Synopsys website

Signed-off-by: Mischa Jonker <mjonker@synopsys.com>
uclibc: arc: Use git instead of website

Signed-off-by: Mischa Jonker <mjonker@synopsys.com>
Revert "arc: Add option for ARC-specific download site"

All packages use github now.

This reverts commit 1445b7fd2e46db5d39ad5e2f5cc262574399f5f3.
Add ARC Xplorer defconfig
dontmerge: add custom ARC u-boot for uimage support

eventually we should from open source, but it's not upstream yet
dontmerge: Add README.md