[AOT] Make native linker name configurable (#20816)

[AOT] Make native linker name configurable (#20816)

Context: https://github.com/android/ndk/wiki/Changelog-r22#announcements
Context: https://github.com/xamarin/xamarin-android/pull/5475

This commit fixes a problem where the AOT compiler hard-codes the native
linker executable name to a platform-specific value but such an
executable does not exist in the toolchain being used, thus making the
AOT compiler fail to link the final binary.

The specific use case here is the new Android NDK r22 which not only
deprecates GNU binutils, but also removes architecture-prefixed `ld`
binary (e.g. `aarch64-linux-android-ld` no longer exists). Instead, the
NDK provides a non-prefixed `ld` binary and two prefixed ones:
`$arch-ld.gold` and `$arch-ld.bfd`. However, since the AOT compiler
hardcodes `ld` as the linker name on Linux systems, the AOT compilation
fails when attempting to link the final executable. Which, in turn,
breaks some Xamarin.Android AOT tests.

This commit fixes the issue by adding a new `ld-name` option to the AOT
compiler allowing one to specify just the name of the linker binary to
use. The rest of the toolchain mechanics doesn't change.