arc: fix defualt toolchain prefix for PIE support

Current arc{be}-elf32- toolchain doesn't support linkage of
position-independent executables (PIE) simply refusing to accept "-pie"
flag.

So we're defaulting to "arc{eb}-buildroot-linux-uclibc-" for now.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>