bootloader & building: macOS: always use Darwin-64bit directory

On macOS, avoid appending -arm suffix to bootloader directory if
we are on arm64 (M1) platform. So instead of Darwin-64bit-arm,
we always use Darwin-64bit. Applies both to bootloader's wscript
as well as frozen application assembly pipeline (or rather,
PyInstaller.PLATFORM).

This is because we now build universal2 bootloaders, so there is
no need for two arch-specific locations. Furthermore, using -arm
suffix for bootloader directory would prevent M1 users from using
pre-compiled bootloaders from git tree (unless we saved one copy
in each location) and in binary wheels (unless we split wheels
into x86_64 and arm64 one).

Overall, considering the current design of x86_64/arm64 support,
it makes more sense to unify the bootloader directory for macOS.

PyInstaller/__init__.py

@@ -67,7 +67,8 @@
 
 
 PLATFORM = compat.system + '-' + compat.architecture
-# Include machine name in path to bootloader for some machines.
-# e.g. 'arm'
-if compat.machine:
+# Include machine name in path to bootloader for some machines (e.g., 'arm').
+# Explicitly avoid doing this on macOS, where we keep universal2 bootloaders
+# in Darwin-64bit folder regardless of whether we are on x86_64 or arm64.
+if compat.machine and not compat.is_darwin:
     PLATFORM += '-' + compat.machine

bootloader/wscript

@@ -747,7 +747,7 @@ def build(ctx):
                                 ctx.env.PYI_SYSTEM + "-" + ctx.env.PYI_ARCH)
     install_path = os.path.normpath(install_path)
 
-    if machine():
+    if machine() and ctx.env.DEST_OS != 'darwin':
         install_path += '-' + machine()
 
     if not ctx.env.LIB_Z: