[Android] Use -l:libunwind.a with --rtlib=compiler-rt

On Android, the unwinder isn't part of the C++ STL and isn't (in older
versions) exported from libc.so. Instead, the driver links the static
unwinder archive implicitly. Currently, the Android NDK implicitly
links libgcc.a to provide both builtins and the unwinder.

To support switching to compiler-rt builtins and libunwind, make
--rtlib=compiler-rt behave the same way on Android, and implicitly pass
-l:libunwind.a to the linker.

Adjust the -ldl logic. For the Android NDK, the unwinder (whether
libgcc.a or libunwind.a) is linked statically and calls a function in
the dynamic loader for finding unwind tables (e.g. dl_iterate_phdr).
On Android, this function is in libc.a for static executables and
libdl.so otherwise, so -ldl is needed. (glibc doesn't need -ldl because
its libc.so exports dl_iterate_phdr.)

Differential Revision: https://reviews.llvm.org/D96403