Documentation for ARC port

Documentation for ARC port

(a) ABI doc:
    https://github.com/foss-for-synopsys-dwc-arc-processors/toolchain/wiki/files/ARCv2_ABI.pdf

(b) Programmer's Reference Manual (PRM) : needs a download request to be filled
    https://www.synopsys.com/dw/ipdir.php?ds=arc-hs44-hs46-hs48
    https://www.synopsys.com/dw/doc.php/ds/cc/programmers-reference-manual-ARC-HS.pdf

As of port merge (and Florian's patch to fix static TLS), glibc testsuite
(cross-test setup) has following failures:

FAIL: elf/tst-audit14
FAIL: elf/tst-audit15
FAIL: elf/tst-audit16
FAIL: elf/tst-ldconfig-ld_so_conf-update
FAIL: elf/tst-libc_dlvsym
FAIL: elf/tst-libc_dlvsym-static
FAIL: iconv/test-iconvconfig		# Needs gconv installed
FAIL: io/ftwtest			# Requires execution by non-root
FAIL: io/tst-lockf
FAIL: libio/tst-wfile-sync
FAIL: locale/tst-localedef-path-norm
FAIL: nptl/test-cond-printers		# needs Python3 and target GDB on target
FAIL: nptl/test-condattr-printers	#    ditto
FAIL: nptl/test-mutex-printers		#    ditto
FAIL: nptl/test-mutexattr-printers	#    ditto
FAIL: nptl/test-rwlock-printers		#    ditto
FAIL: nptl/test-rwlockattr-printers	#    ditto
FAIL: nptl/tst-umask1			# passes if run natively on target (NFS ACLv3 support needed)
FAIL: nss/bug-erange
FAIL: nss/tst-nss-files-hosts-getent
FAIL: nss/tst-nss-files-hosts-multi
FAIL: posix/bug-ga2
FAIL: posix/globtest			# require same user on target and host
FAIL: posix/tst-getaddrinfo5
FAIL: stdio-common/tst-vfprintf-width-prec
FAIL: stdio-common/tst-vfprintf-width-prec-alloc
FAIL: stdio-common/tst-vfprintf-width-prec-mem
FAIL: string/tst-strerror
FAIL: string/tst-strsignal
FAIL: sunrpc/bug20790			# missing cpp on target
FAIL: timezone/tst-tzset

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>