uclibc: add explicit setup of ARC-specific options

As with other architectures for ARC we need to have an ability to set
specific options in uClibc.

In particular this is required for selection of ARC ISA version.

[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 <akolesov@synopsys.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>