toolchain: fix URLs for ARC gcc/binutils/gdb

toolchain: fix URLs for ARC gcc/binutils/gdb

In commit cd9d58f1fc285bcc09f1e0e29ee300a8a9053053 ("toolchain: bumb
ARC tools to arc-2018.09 release"), the ARC-specific binutils, gcc and
gdb versions were updated to use the 2018.09 release. However, they
are mistakenly pointing to a branch rather than a tag. This commit
fixes that by using the proper release tag.

Signed-off-by: Evgeniy Didin <Evgeniy.Didin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: arc-buildroot@synopsys.com
[Thomas: rework commit log]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>