_build.sh: fixup machine arch name for macOS [ci skip]

`uname -m` returns `arm64` on macOS ARM64 (not `aarch64`). These scripts
expect `aarch64` (universally on all platforms). Instead of tweaking all
of those to `arm64` for macOS, rewrite the detected value to `aarch64`.

Host triplets seems to work with the latter also.

This fixes to actually run the ARM64 curl binary on ARM64 machines.

_build.sh

@@ -197,6 +197,11 @@ export ZIP=
 
 unamem="$(uname -m)"
 
+# for macOS ARM64 hosts
+if [ "${unamem}" = 'arm64' ]; then
+ unamem='aarch64'
+fi
+
 readonly _LOG='logurl.txt'
 readonly _SELF='curl-for-win'
 if [ -n "${APPVEYOR_ACCOUNT_NAME:-}" ]; then