Fix macOS host + linux-bionic-arm64 target cross-compile build

[jonpryor]

Context: dotnet/runtime#101727

Previously, trying to use dotnet publish from a macOS host would result in obscure build failures, e.g.

% dotnet publish -c Release
…
clang : error : invalid linker name in argument '-fuse-ld=lld'

Apply the workaround from dotnet/runtime#101727 to workaround this. The $(ANDROID_NDK_HOME) environment variable requuired:

# we're all xamarin-android devs here, right? ;-)
% export ANDROID_NDK_HOME=$HOME/android-toolchain/ndk
% dotnet publish -c Release
# Creates DotNet/bin/Release/net8.0/linux-bionic-arm64/publish/libdotnet.so

[jonpryor]

lol, so…

While libdotnet.so builds on macOS, the resulting binary is not usable as-is. Trying to use the resulting libdotnet.so on-device results in:

E AndroidRuntime: java.lang.UnsatisfiedLinkError: Unable to load native library "/data/app/~~4oPnKwAB0bVVIZs6TwgFWg==/com.jonathanpeppers.nativeaot-ZfYIxey30jLo8YEB5UVaPw==/lib/arm64/libnativeaot.so": dlopen failed: libra
ry "libc++_shared.so" not found: needed by /data/app/~~4oPnKwAB0bVVIZs6TwgFWg==/com.jonathanpeppers.nativeaot-ZfYIxey30jLo8YEB5UVaPw==/lib/arm64/libdotnet.so in namespace clns-4

This is because libdotnet.so has a library reference to libc++_shared.so, which doesn't exist.

Fortunately this is "easy" to solve: copy libc++_shared.so from the NDK into your project:

% mkdir  Native/app/src/main/jniLibs/arm64-v8a
% cp $ANDROID_NDK_HOME/toolchains/llvm/prebuilt/darwin-x86_64/sysroot/usr/lib/aarch64-linux-android/libc++_shared.so \
    Native/app/src/main/jniLibs/arm64-v8a

Build, install, run…

% ./gradlew assembleRelease
% adb install app/build/outputs/apk/release/app-release.apk
% adb shell am start com.jonathanpeppers.nativeaot/android.app.NativeActivity

…and it crashes.

I think the NativeAOT + macOS host + linux-bionic toolchain is broken:

F DEBUG   : *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
F DEBUG   : Build fingerprint: 'google/raven/raven:14/AP1A.240305.019.A1/11445699:user/release-keys'
F DEBUG   : Revision: 'MP1.0'
F DEBUG   : ABI: 'arm64'
F DEBUG   : Timestamp: 2024-05-01 11:01:48.710687354-0400
F DEBUG   : Process uptime: 1s
F DEBUG   : Cmdline: com.jonathanpeppers.nativeaot
F DEBUG   : pid: 10516, tid: 10516, name: ppers.nativeaot  >>> com.jonathanpeppers.nativeaot <<<
F DEBUG   : uid: 10371
F DEBUG   : tagged_addr_ctrl: 0000000000000001 (PR_TAGGED_ADDR_ENABLE)
F DEBUG   : signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0x0000000000000006
F DEBUG   : Cause: null pointer dereference
F DEBUG   :     x0  0000000000000006  x1  0000007fda6f3340  x2  0000007eb6852980  x3  0000007fda6ef330
F DEBUG   :     x4  0000007eb6852a00  x5  0000000000000004  x6  0000000001414d4c  x7  0000007eb6852004
F DEBUG   :     x8  0000007eb6852980  x9  0000000000000006  x10 000000000000000f  x11 757274736e6f6320
F DEBUG   :     x12 676e696c6c61632c  x13 000180000045dfd0  x14 0000000000018510  x15 0000000000000000
F DEBUG   :     x16 0000007eb7b3d680  x17 0000007eb7a5fb00  x18 0000007eb6d90000  x19 0000007fda6f3340
F DEBUG   :     x20 0000007eb6852980  x21 0000000000000006  x22 0000007eb7b4b000  x23 0000007eb67c9e70
F DEBUG   :     x24 0000007fda6f0518  x25 0000000000000022  x26 0000007eb6843c00  x27 0000007fda6f0500
F DEBUG   :     x28 0000007eb7b49000  x29 0000007fda6ef2e0
F DEBUG   :     lr  0000007b57aebc80  sp  0000007fda6ef2c0  pc  0000007b57aebcb0  pst 0000000080001000
F DEBUG   : 1 total frames
F DEBUG   : backtrace:
F DEBUG   :       #00 pc 00000000000c1cb0  /data/app/~~y5LQdnO4d1iwNk2Z6I4Jpg==/com.jonathanpeppers.nativeaot-HJbM4Jw2CNRiWe4-Rn2IxA==/lib/arm64/libdotnet.so (BuildId: 21c56035e6fc411dab6ab373715ba35e82c0fb5f)