lib/raid6/test/Makefile: Use $(pound) instead of \# for Make 4.3

lib/raid6/test/Makefile: Use $(pound) instead of \# for Make 4.3

Buidling raid6test on Ubuntu 21.10 (ppc64le) with GNU Make 4.3 shows the
errors below:

    $ cd lib/raid6/test/
    $ make
    <stdin>:1:1: error: stray ‘\’ in program
    <stdin>:1:2: error: stray ‘#’ in program
    <stdin>:1:11: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ \
        before ‘<’ token

    [...]

The errors come from the HAS_ALTIVEC test, which fails, and the POWER
optimized versions are not built. That’s also reason nobody noticed on the
other architectures.

GNU Make 4.3 does not remove the backslash anymore. From the 4.3 release
announcment:

> * WARNING: Backward-incompatibility!
>   Number signs (#) appearing inside a macro reference or function invocation
>   no longer introduce comments and should not be escaped with backslashes:
>   thus a call such as:
>     foo := $(shell echo '#')
>   is legal.  Previously the number sign needed to be escaped, for example:
>     foo := $(shell echo '\#')
>   Now this latter will resolve to "\#".  If you want to write makefiles
>   portable to both versions, assign the number sign to a variable:
>     H := \#
>     foo := $(shell echo '$H')
>   This was claimed to be fixed in 3.81, but wasn't, for some reason.
>   To detect this change search for 'nocomment' in the .FEATURES variable.

So, do the same as commit 9564a8cf422d ("Kbuild: fix # escaping in .cmd
files for future Make") and commit 929bef467771 ("bpf: Use $(pound) instead
of \# in Makefiles") and define and use a $(pound) variable.

Reference for the change in make:
https://git.savannah.gnu.org/cgit/make.git/commit/?id=c6966b323811c37acedff05b57

Cc: Matt Brown <matthew.brown.dev@gmail.com>
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Song Liu <song@kernel.org>