Fix default values for AVX and AVX512 OS state enabled flags

AVX and AVX512 strictly require the OS to enable OXSAVE
otherwise we can face "illegal instruction" depending on the context state

Author: mhandb

include/xsimd/config/xsimd_cpuid.hpp

@@ -197,8 +197,11 @@ namespace xsimd
                 // https://docs.kernel.org/admin-guide/hw-vuln/gather_data_sampling.html
 
                 unsigned sse_state_os_enabled = 1;
-                unsigned avx_state_os_enabled = 1;
-                unsigned avx512_state_os_enabled = 1;
+                // AVX and AVX512 strictly require OSXSAVE to be enabled by the OS.
+                // If OSXSAVE is disabled (e.g., via bcdedit /set xsavedisable 1),
+                // AVX state won't be preserved across context switches, so AVX cannot be used.
+                unsigned avx_state_os_enabled = 0;
+                unsigned avx512_state_os_enabled = 0;
 
                 // OSXSAVE: A value of 1 indicates that the OS has set CR4.OSXSAVE[bit
                 // 18] to enable XSETBV/XGETBV instructions to access XCR0 and