add definition for aarch64

[furushchev]

With this PR, compile errors on arm64 architecture is fixed. The error occurs since LINUX_ARM is not defined on aarch64 (=arm64).

Scanning dependencies of target wirehair
[  4%] Building CXX object CMakeFiles/wirehair.dir/wirehair.cpp.o
In file included from /tmp/wirehair/WirehairTools.h:34:0,
                 from /tmp/wirehair/WirehairCodec.h:211,
                 from /tmp/wirehair/wirehair.cpp:30:
/tmp/wirehair/gf256.h:70:14: fatal error: tmmintrin.h: No such file or directory
     #include <tmmintrin.h> // SSSE3: _mm_shuffle_epi8
              ^~~~~~~~~~~~~
compilation terminated.
CMakeFiles/wirehair.dir/build.make:62: recipe for target 'CMakeFiles/wirehair.dir/wirehair.cpp.o' failed
make[2]: *** [CMakeFiles/wirehair.dir/wirehair.cpp.o] Error 1
CMakeFiles/Makefile2:104: recipe for target 'CMakeFiles/wirehair.dir/all' failed
make[1]: *** [CMakeFiles/wirehair.dir/all] Error 2
Makefile:129: recipe for target 'all' failed
make: *** [all] Error 2

[catid]

Prefer doing this in the C++ code but seems harmless