--emit=exe and --emit=dll link for Android

[ASDAlexander77]

What

A triple <arch>-linux-android<api> now links with --emit=exe and --emit=dll, through the Android NDK (--android-ndk-path, or ANDROID_NDK_HOME). --emit=dll is what an app loads with System.loadLibrary; --emit=exe shares the same code and suits adb shell runs.

tslang --emit=dll -mtriple=aarch64-linux-android29 --opt -mm=gc ^
    --default-lib-path=<DefaultLib>\__build\android\arm64-v8a ^
    --gc-lib-path=3rdParty\gc\android\arm64-v8a\release\lib ^
    --tslang-lib-path=__build\tslang-runtime\release\android\arm64-v8a ^
    mylib.ts -o libmylib.so

How

• NDK: the clang driver gets --sysroot and -resource-dir from the NDK. The host tag (windows-x86_64, linux-x86_64, …) and the clang version are found by listing directories, not hardcoded. From those, the driver itself picks Bionic's startup objects and libraries for the API level, compiler-rt's builtins and libunwind.a; tslang's clang 22 driver handles the NDK's clang 21 layout.
• PIC by default for Android, in both obj.cpp and buildExe: executables must be PIE, and LLVM's default is static. -relocation-model=pic is no longer needed.
• One self-contained binary. Static libc++ + libc++abi (-l:libc++.a). No -lpthread or -lrt (part of Bionic's libc) and no -lcurl (the Android default library uses http_stub.cpp). --emit=dll links the static default library: the shared one leaves GC_* to the hosting process, and an app's process (the Java VM) has no collector. The output depends only on libc.so, libm.so and libdl.so.
• --no-undefined for an Android .so: a symbol Bionic lacks fails the link, not the app's System.loadLibrary.
• Clear errors for a triple with no API level (the driver would silently take the oldest sysroot), no NDK, or a directory that is not an NDK.
• checkGCLibPath/checkTslangLibPath name the library as the target does (libgc.a), not the host (gc.lib). That's the same bug class as Codegen picks the target's CRT, not the host's (Android cross-compile) #506.

Verification

• Through tslang, for arm64-v8a and x86_64, under gc and none, both exe and dll:
  • all link;
  • the exe is PIE with /system/bin/linker64 as its interpreter;
  • the dependencies are libc, libm and libdl;
  • no strong undefined symbols remain against the API 29 Bionic stubs;
  • the dll exports the TS export function by its plain name and has .init_array (top-level code runs at load).
• A debug (--di) dll links with DWARF.
• New test-compile-android-codegen runs everywhere, with no NDK: it checks Android code is position independent without -relocation-model=pic, and the API-level and missing-NDK errors. The PIC check discriminates: x86_64 Linux code without PIC takes the same constant as an absolute immediate, and the test asserts that too.
• New test-compile-android-link links an exe and a .so for x86_64/gc. It's skipped (not failed) without ANDROID_NDK_HOME and the Android builds from Android: build scripts for the collector and the async runtime #507 and DefaultLib Cannot build config tsc debug: MLIR not found #22.
• Full Release suite on Windows (with the NDK set): 3855/3855 passed.
• Link-only: nothing has run on a device or emulator. -mm=rc through tslang wasn't exercised (only through the earlier manual NDK link).

Known limitation: -mm=gc in an app library

GC_init and GC_enable_threads are injected into the module's global constructor (or main), so a gc .so with top-level code starts its collector at load; without either, Boehm initializes lazily. But Boehm only scans the stacks of threads registered with it. A JVM thread calling into the library (any JNI thread other than the one that loaded it) isn't registered, so under -mm=gc objects allocated or held there aren't safe yet. -mm=rc and -mm=none aren't affected. This needs a device or emulator run to confirm and fix.

Follow-ups

• The DefaultLib README update showing this (ASDAlexander77/TypeScriptCompilerDefaultLib, branch android-readme-tslang-link), after this merges.
• A CI job building the Android pieces, then a release (v0.0-pre-alpha89 predates Codegen picks the target's CRT, not the host's (Android cross-compile) #506).

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