ARM SME detection for macOS

This is a follow-up to !385 (merged), which only implemented support for Streaming SVE / SME on Linux. In the meantime, Macs with suitable CPUs have been released, so let's add the detection code and compiler flags for it. This revealed two bugs (SME shouldn't be the highest, i.e. default, vector ISA, as this will break instruction_set="best" and test_aligned_and_nt_stores which is intentionally only run on one ISA; streaming attribute must not be added to Neon kernels).

src/pystencils/backends/simd_instruction_sets.py

 import os
 import platform
-from ctypes import CDLL
+from ctypes import CDLL, c_int, c_size_t, sizeof, byref
 from warnings import warn
 
 import numpy as np
@@ -38,7 +38,14 @@ def get_supported_instruction_sets():
     if 'PYSTENCILS_SIMD' in os.environ:
         return os.environ['PYSTENCILS_SIMD'].split(',')
     if platform.system() == 'Darwin' and platform.machine() == 'arm64':
-        return ['neon']
+        result = ['neon']
+        libc = CDLL('/usr/lib/libc.dylib')
+        value = c_int(0)
+        size = c_size_t(sizeof(value))
+        status = libc.sysctlbyname(b"hw.optional.arm.FEAT_SME", byref(value), byref(size), None, 0)
+        if status == 0 and value.value == 1:
+            result.insert(0, "sme")
+        return result
     elif platform.system() == 'Windows' and platform.machine() == 'ARM64':
         return ['neon']
     elif platform.system() == 'Linux' and platform.machine() == 'aarch64':
@@ -59,7 +66,7 @@ def get_supported_instruction_sets():
                 length //= 2
             result.append(name)
         if hwcap2 & (1 << 23):  # HWCAP2_SME
-            result.append("sme")
+            result.insert(0, "sme")  # prepend to list so it is not automatically chosen as best instruction set
         return result
     elif platform.system() == 'Linux' and platform.machine().startswith('riscv'):
         libc = CDLL('libc.so.6')