Adding better support for avx- and avx2-class instruction sets.

-march=sandybridge enables MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES and PCLMUL instruction set support. See https://gcc.gnu.org/onlinedocs/gcc-5.4.0/gcc/x86-Options.html#x86-Options

-march=haswell enables MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2 and F16C instruction set support. See https://gcc.gnu.org/onlinedocs/gcc-5.4.0/gcc/x86-Options.html#x86-Options