It's better to clip to the existing versions here rather than build
the lower levels as if they existed.
cc.Module's begin() will now normalize the sdk_version to a valid
value for the given architecture so that doesn't have to be checked
later.
Test: make checkbuild with all other libc ndk_library changes
Change-Id: I2fa834f7a1ac3b03a0723291778d86207fcfda72