Support Android NDK

[meme]

• Fixes linking against Android's libc-variant, bionic
• Producing executables and shared objects that link against libc now link without trouble, provided you use a proper libc.txt

You can test this for yourself by downloading the latest Android NDK at https://developer.android.com/ndk/downloads/ and creating a libc.txt for AArch64, for example (of course directed to where your NDK is) then running zig build-exe:

include_dir=/opt/android-ndk-r20/toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/include
sys_include_dir=/opt/android-ndk-r20/toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/include
static_crt_dir=/opt/android-ndk-r20/toolchains/llvm/prebuilt/linux-x86_64/lib/gcc/aarch64-linux-android/4.9.x/
crt_dir=/opt/android-ndk-r20/toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/lib/aarch64-linux-android/21

msvc_lib_dir=
kernel32_lib_dir=

(note that this example uses Android SDK version 21)

$ zig build-exe -target aarch64v8-linux-android --library c --libc aarch64_android_21.txt main.zig

Note that you must pass the path to the gcc runtime, or else you will get errors regarding missing -lgcc -- @andrewrk, can we do away with linking against libgcc on Android entirely?

The same steps above may also be used for producing a static archive, or shared object -- producing statically compiled Android executables is broken with these changes, should be fixed before merge

[andrewrk]

can we do away with linking against libgcc on Android entirely?

Yes, we don't need -lgcc because zig provides libcompiler_rt.a.

[andrewrk]

Looking good, just a few tweaks and I think it's ready to merge.

[andrewrk]

Suggested change

	if (target_requires_pie(g->zig_target) && !is_dyn_lib && g->libc != nullptr) {
	if (target_requires_pie(g->zig_target) && g->out_type == OutTypeExe) {

[andrewrk]

Instead of the logic below, shouldn't it be here?

if (g->have_dynamic_link) {
    crt1o = "crtbegin_dynamic.o";
} else {
    crt1o = "crtbegin_static.o";
}

Then the other logic remains unchanged:

         } else if (!g->have_dynamic_link) { 
             crt1o = "crt1.o";
         }

[andrewrk]

What's the reasoning for g->libc != nullptr? I don't think it's necessary - even if you were running zig compiler on android you'd want crtend_android.o on the link line, right? g->libc is just if someone provided a custom libc.txt file with paths.

I think you want g->link_in_crt instead (see the below if statement). This is whether we want to link against libc.