Android: Use CMAKE_SYSROOT for Android sysroot

[etcwilde]

The NDK configures CMAKE_SYSROOT in android-legacy.toolchain.cmake. I've verified this from Android NDK r26b to r29.

This gets the build system cross-compiling to Android from a non-Windows builder.

[etcwilde]

@swift-ci please test

[etcwilde]

The Windows bits don't appear to be using the Android NDK toolchain file, which might be why CMAKE_SYSROOT wasn't getting set up. I suspect that this won't pass as-is. I may change build.ps1 here a bit to follow the Google documentation, which is how I've been testing/building this: https://developer.android.com/ndk/guides/cmake.

[etcwilde]

Linux test failure:

********************
Failed Tests (1):
  lldb-api :: lang/swift/expression/no_calculator/TestSwiftNoProcess.py

This is not affected by the Android build.

macOS failure was a clang segfault while building parts of compiler-rt for the iOS simulator:

FAILED: compiler-rt/lib/asan/CMakeFiles/RTAsan_dynamic.iossim.dir/asan_thread.cpp.o 

Also unrelated to changes to Android.

[etcwilde]

@swift-ci please test

[etcwilde]

@swift-ci please test

[compnerd]

I definitely like the removal of --sysroot from C/CXX FLAGS

[etcwilde]

The Windows failure is unrelated and should have been fixed by swiftlang/swift-build#1236

[etcwilde]

@swift-ci please test Windows

[finagolfin]

The pre-commit Windows CI do nothing with Android, so if you want to test this, I suggest you temporarily add the -Android flag in the batch file and run the Windows toolchain CI command.

That linked pull is so we can add building Android SDKs to the pre-commit Windows CI and avoid manual testing workarounds like this.

[etcwilde]

The pre-commit Windows CI do nothing with Android

Yeah, I know. I'm just trying to get this merged.

[etcwilde]

@swift-ci please test macOS

[finagolfin]

I'm just worried this may break the trunk Windows snapshot toolchain CI, which adds that flag I linked and builds the Android SDKs but only after these pulls are merged, so suggesting this be tested first.

[etcwilde]

@swift-ci please test macOS

[finagolfin]

I hope you weren't waiting for me to sign off on this, Evan. All I meant last month is that the current pre-merge Windows CI do not build the Android SDKs, #87867, so I have been testing Android changes like this on Windows by modifying that batch file and running the PR toolchain CI, then removing that flag before merging.

I was simply suggesting you do the same, but not a problem at all if you don't, as the post-merge Windows trunk snapshot toolchain CI will give us that signal anyway.

[etcwilde]

Ah, okay. I tested it on my Windows box locally. I'll keep an eye out for failures on Windows from this.