arc: Merge ARCv2 string routines in generic ARC .S files

arc: Merge ARCv2 string routines in generic ARC .S files

In cde74b83f9b2 "ARC: remove special CFLAGS/LDFLAGS handling" we
got rid of CONFIG_ARC_CPU_HS which was used to select ARCv2-specific
implementation of optimized string routines. So now ARCv2-tuned
memset/memcpy/strcmp are not used, instead those for ARC700 used for
both ARC700 and ARCHS.

Without uClibc config option we may only tell which CPU type we're
targeting by built-in defines of GCC. I.e. no more conditional file
inclusion in Makefiles. That leaves us only one option - merge both
implementations in 1 file and use ifdefs.

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