perf: Disable perf for ARC770 only, enable for ARC HS38

perf: Disable perf for ARC770 only, enable for ARC HS38

Toolchain built for ARCv1 (read for ARC700 cores) by default has
disabled atomic ops (-mno-atomic). When we build Linux kernel for ARC770
which has LL/SC instructions and thus may handle normally atomic ops we
explicitly add "-matomic" in CFLAGS. But since user-space perf utility has
no way to extract CPU config options from Kconfig/defconfig it uses
compiler default settings.

In case of ARCv2 (read ARC HS38) atomics are enabled by default and so
perf builds perfectly fine thus reenabling perf for ARC HS38 (actually
for non-ARC700 targets).

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