libc++ linker scripts incorrect for LP32 API 21+

[DanAlbert]

Since we only ship one libc++ linker script set (one shared, one static) in the NDK, this script no longer works for the case where libandroid_support is not used for the target API level. make_standalone_toolchain.py won't copy libandroid_support for android-21+ since it is not needed, but the LP32 linker scripts assume it is needed because it is for earlier API levels.

I think the right solution here is to either ship multiple linker scripts (libc++.so.16, libc++.so.17, etc) for each supported API level, or to just remove them from the NDK distribution and let make_standalone_toolchain.py generate them.

Since we're so late in the r17 release cycle, I don't think we should alter the existing linker scripts since it seems likely that non-standalone toolchain users of them already have libandroid_support on the path, and in the short term we should avoid breaking their use case since the downsides of linking libandroid_support when it is not needed are small.

For r17, we'll switch to having make_standalone_toolchain.py generating its own linker scripts but also preserve the existing linker scripts in the NDK. For r18 we'll either remove them or generate one per API level.

[DanAlbert]

While uglier implementation-wise than generating fresh linker scripts, I think we're better off having make_standalone_toolchain.py do a rewrite of the linker scripts after copying them since that will avoid issues where we update one template and forget to update the other.

[DanAlbert]

https://android-review.googlesource.com/c/platform/ndk/+/665595 is the r17 fix for this. Moving the cleanup to r18.

[DanAlbert]

Cleanup merged into master for r18. There's now a set of libc++.so.$API_LEVEL scripts.