[Build] Derive TARGET_ARCH from -arch argument in make_win64.mak

[HannesWell]

This allows cross-compilation on Windows between x86_64 and aarch64 by specifying the -arch argument as usual when calling the win32/build.bat script.

@chirontt do you see any problem with this?

[github-actions]

Test Results

  660 files  ±0    660 suites  ±0   1h 11m 44s ⏱️ -2s
2 195 tests ±0  2 147 ✅  - 1   47 💤 ±0  1 ❌ +1 
6 729 runs  ±0  6 585 ✅  - 1  143 💤 ±0  1 ❌ +1 

For more details on these failures, see this check.

Results for commit 73b0a0c. ± Comparison against base commit 343e816.

♻️ This comment has been updated with latest results.

[chirontt]

Sounds good to me to re-use the -arch argument for cross-compiling purpose. This removes the need to set a TARGET_ARCH environment variable for cross-compiling, as originally intended.

But cross-compiling also requires a target JDK for the relevant headers and/or libraries suitable for the target architecture. Can the -java argument be used for this purpose?

[HannesWell]

Sounds good to me to re-use the -arch argument for cross-compiling purpose. This removes the need to set a TARGET_ARCH environment variable for cross-compiling, as originally intended.

Exactly. And it also verifies that the jdk passed with the -java argument exists.

But cross-compiling also requires a target JDK for the relevant headers and/or libraries suitable for the target architecture. Can the -java argument be used for this purpose?

That's also right and yes the -java argument can be used for that.
I'm currently enhancing the documentation about how to build the Equinox binaries in #640 and mentioned in there as an example.

[HannesWell]

Build is green now (besides the usual TCK failure and an occasional DS test failure).