sse should be used only on x86_64

[martin-g]

Related to #21

Trying to build on Linux ARM64 leads to the following error:

g++ -static-libgcc -static-libstdc++ -D__cplusplus=201103L -I"./src/" -I"/usr/include/" -I"codebase/seqlib/src/libs/seqan-library-2.0.1/include" -I"codebase/seqlib/src/libs/libdivsufsort-2.0.1-64bit/" -Icodebase/argumentparser/src -Icodebase/seqlib/src -Icodebase/gindex/src -DRELEASE_VERSION -g -O3 -fdata-sections -ffunction-sections -c -fmessage-length=0 -ffreestanding -fopenmp -std=c++11 -Werror=return-type -pthread  -o obj_linux/src/ksw2/ksw2_ll_sse.o src/ksw2/ksw2_ll_sse.cc
src/ksw2/ksw2_ll_sse.cc:4:10: fatal error: emmintrin.h: No such file or directory
 #include <emmintrin.h>
          ^~~~~~~~~~~~~
compilation terminated.
make: *** [Makefile:124: obj_linux/src/ksw2/ksw2_ll_sse.o] Error 1

SSE and AVX are x86_64 specific features.

Could I please request adding support for Linux ARM64 ?

[martin-g]

Will be fixed with #23