Fix: remove hard-coded x86 flags in Makefile

[alessandroamella]

The hard-coded flags -mprefer-vector-width=256 and -mno-avx512f inside Makefile caused build failures on non-x86 architectures such as ARMv8.

This change allows the project to build correctly on multiple architectures (tested on ARMv8 / Apple Silicon on Asahi Linux), by removing the hard-coded x86-specific compiler flags and adding a conditional check so they are only applied on x86 architectures:

# Flag specifiche per architetture x86
ifeq ($(findstring x86,$(ARCH)),x86)
    CXXFLAGS += -mprefer-vector-width=256 -mno-avx512f
endif