_build.sh: keep out arm64e designator from autotools [ci skip]

autotools doesn't recognize `arm64e`:
```
Invalid configuration `arm64e-apple-darwin': machine `arm64e-apple' not recognized
```
Ref: https://github.com/curl/curl-for-win/actions/runs/6530947363/job/17732106776#step:3:4161

So backtrack and pass this value only to `-arch` for clang-apple,
and `CMAKE_OSX_ARCHITECTURES` (we will adjust if that fails, but
so far it seems to be accepted). This also revert passing arm64e
to standard llvm which accepts this, but seems to be ignoring it
anyway (as of llvm 16/17).

Regression from 898aeab

_build.sh

@@ -523,12 +523,13 @@ build_single_target() {
   [ "${_CPU}" = 'a64' ] && _machine='aarch64'
 
   if [ "${_OS}" = 'mac' ] && [ "${_machine}" = 'aarch64' ] && [ "${_CC}" = 'llvm' ]; then
-    _machine='arm64e'
+    # llvm-apple supports multiple archs separated by ';', e.g. 'arm64e;x86_64'
+    # It also understands arm64e (vs arm64)
+    _machines='arm64e'
+  else
+    _machines="${_machine}"
   fi
 
-  # llvm-apple supports multiple archs separated by ';', e.g. 'arm64e;x86_64'
-  _machines="${_machine}"
-
   export _CURL_DLL_SUFFIX=''
   export _CURL_DLL_SUFFIX_NODASH=''
   if [ "${_OS}" = 'win' ]; then