lib/raid6/test: fix build on distros whose /bin/sh is not bash

You can build a user-space test program for the raid6 library code,
like this:

  $ cd lib/raid6/test
  $ make

The command in $(shell ...) function is evaluated by /bin/sh by default.
(or, you can specify the shell by passing SHELL=<shell> from command line)

Currently '>&/dev/null' is used to sink both stdout and stderr. Because
this code is bash-ism, it only works when /bin/sh is a symbolic link to
bash (this is the case on RHEL etc.)

This does not work on Ubuntu where /bin/sh is a symbolic link to dash.

I see lots of

  /bin/sh: 1: Syntax error: Bad fd number

and

  warning "your version of binutils lacks ... support"

Replace it with portable '>/dev/null 2>&1'.

Fixes: 4f8c55c ("lib/raid6: build proper files on corresponding arch")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

lib/raid6/test/Makefile

@@ -35,13 +35,13 @@ endif
 ifeq ($(IS_X86),yes)
         OBJS   += mmx.o sse1.o sse2.o avx2.o recov_ssse3.o recov_avx2.o avx512.o recov_avx512.o
         CFLAGS += $(shell echo "pshufb %xmm0, %xmm0" |		\
-                    gcc -c -x assembler - >&/dev/null &&	\
+                    gcc -c -x assembler - >/dev/null 2>&1 &&	\
                     rm ./-.o && echo -DCONFIG_AS_SSSE3=1)
         CFLAGS += $(shell echo "vpbroadcastb %xmm0, %ymm1" |	\
-                    gcc -c -x assembler - >&/dev/null &&	\
+                    gcc -c -x assembler - >/dev/null 2>&1 &&	\
                     rm ./-.o && echo -DCONFIG_AS_AVX2=1)
 	CFLAGS += $(shell echo "vpmovm2b %k1, %zmm5" |          \
-		    gcc -c -x assembler - >&/dev/null &&        \
+		    gcc -c -x assembler - >/dev/null 2>&1 &&	\
 		    rm ./-.o && echo -DCONFIG_AS_AVX512=1)
 else ifeq ($(HAS_NEON),yes)
         OBJS   += neon.o neon1.o neon2.o neon4.o neon8.o recov_neon.o recov_neon_inner.o