gcc: Apply multilib fix to ARC as well

W/o that hack target GCC assume existence of per-mcpu folders,
which are missing.

In particular G++ failed to find "bits/c++config.h":
------------------>8------------------
 root@hsdk:~# cat test.cc
 #include <cstdlib>

 int myfunc(void)
 {
 }

 root@hsdk:~# g++ -c test.cc -v
 Using built-in specs.
 COLLECT_GCC=g++
 Target: arc-oe-linux
 Configured with: ../../../../../../work-shared/gcc-11.1.0-r0/gcc-11.1.0/configure --build=x86_64-linux --host=arc-oe-linux --target=arc-oe-linux --prefix=/usr --exec_prefix=/usr -x
 Thread model: posix
 Supported LTO compression algorithms: zlib
 gcc version 11.1.1 20210523 (GCC)
 COLLECT_GCC_OPTIONS='-c' '-v' '-shared-libgcc' '-mcpu=hs38_linux'
  /usr/libexec/gcc/arc-oe-linux/11.1.1/cc1plus -quiet -v -imultilib hs38_linux -D_GNU_SOURCE test.cc -quiet -dumpbase test.cc -dumpbase-ext .cc -mcpu=hs38_linux -version -o /tmp/ccs
 GNU C++17 (GCC) version 11.1.1 20210523 (arc-oe-linux)
         compiled by GNU C version 11.1.1 20210523, GMP version 6.2.1, MPFR version 4.1.0, MPC version 1.2.1, isl version none
 GGC heuristics: --param ggc-min-expand=99 --param ggc-min-heapsize=129242
 ignoring nonexistent directory "/usr/lib/gcc/arc-oe-linux/11.1.1/../../../../include/c++/11.1.1/arc-oe-linux/hs38_linux"
 ignoring nonexistent directory "/usr/lib/arc-oe-linux/11.1.1/include"
 ignoring nonexistent directory "/usr/local/include"
 ignoring nonexistent directory "/usr/lib/gcc/arc-oe-linux/11.1.1/../../../../arc-oe-linux/include"
 #include "..." search starts here:
 #include <...> search starts here:
  /usr/lib/gcc/arc-oe-linux/11.1.1/../../../../include/c++/11.1.1
  /usr/lib/gcc/arc-oe-linux/11.1.1/../../../../include/c++/11.1.1/backward
  /usr/lib/gcc/arc-oe-linux/11.1.1/include
  /usr/lib/gcc/arc-oe-linux/11.1.1/include-fixed
  /usr/include
 End of search list.
 GNU C++17 (GCC) version 11.1.1 20210523 (arc-oe-linux)
         compiled by GNU C version 11.1.1 20210523, GMP version 6.2.1, MPFR version 4.1.0, MPC version 1.2.1, isl version none
 GGC heuristics: --param ggc-min-expand=99 --param ggc-min-heapsize=129242
 Compiler executable checksum: 6df2f07a822bfbbb80a61414b712b75d
 In file included from test.cc:1:
 /usr/include/c++/11.1.1/cstdlib:41:10: fatal error: bits/c++config.h: No such file or directory
    41 | #include <bits/c++config.h>
       |          ^~~~~~~~~~~~~~~~~~
 compilation terminated.
------------------>8------------------

Note "ignoring nonexistent directory "/usr/lib/gcc/arc-oe-linux/11.1.1/../../../../include/c++/11.1.1/arc-oe-linux/hs38_linux"
message which is being used by GCC due to the fact of implicit "-mcpu=hs38_linux".

In fact this header "bits/c++config.h" is located in "/usr/lib/gcc/arc-oe-linux/11.1.1/../../../../include/c++/11.1.1/arc-oe-linux"
on target.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>