Merge build_darwin_arm64.sh and build_darwin_arm64.sh

Once #155 and #156 are resolved, the two build scripts will be identical.

At the moment, the only differences are:
```
--- build_darwin_arm64.sh 2025-10-02 11:07:02.779859613 -0700
+++ build_darwin_x86_64.sh 2025-10-02 11:42:00.835147638 -0700
@@ -1,7 +1,7 @@
#!/bin/sh -eu

-BUILD_CPU=arm64
-BUILD_HOST=arm-apple-darwin
+BUILD_CPU=x86_64
+BUILD_HOST=x86_64

echo "running build_darwin_$BUILD_CPU.sh..."

@@ -78,7 +78,7 @@
--host=$BUILD_HOST \
--disable-clock-gettime \
--with-pic
-make ${jobs:+-j${jobs}} && make ${jobs:+-j${jobs}} check && make install
+make ${jobs:+-j${jobs}} && make install
cd ../../

tar -xvzf "tor-$TOR_VERSION.tar.gz" -C $BUILD_CPU
@@ -105,5 +105,5 @@
--disable-tool-name-check \
ac_cv_func_getentropy=no \
ac_cv_func_clock_gettime=no
-make ${jobs:+-j${jobs}} && make install
+make ${jobs:+-j${jobs}} && make ${jobs:+-j${jobs}} check && make install
cd ../../
```

After #155 and #156 are resolved, compare build_darwin_arm64.sh and build_darwin_x86_64.sh using the differences shown in this issue. Consolidate the identical build logic while preserving the architecture-specific values and checks, then verify that the resulting scripts remain equivalent for both Darwin targets.

Written by the indexing model from the issue text.