Fix libdl dependency on Android and remove librt hack.

Fix libdl dependency on Android and remove librt hack.

The libdl library is already included on target builds of Android and needs
to be added to the build command line with a particular order to avoid
undefined references in other libraries. Fix this by only explicitly including
it in host builds and relying on the implicit inclusion on target builds.

Also remove the librt hack which is not longer necessary due to the AOSP build
bot having been removed.

BUG=chromium:469973
LOG=Y

Review URL: https://codereview.chromium.org/1036133005

Cr-Commit-Position: refs/heads/master@{#27535}