Do it more simiply. This also fixes single target builds.

[before]

  $ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- arch/arm64/crypto/aes-glue-ce.i
    [snip]
  make[4]: *** No rule to make target 'arch/arm64/crypto/aes-glue-ce.i'.  Stop.

[after]

  $ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- arch/arm64/crypto/aes-glue-ce.i
    [snip]
    CPP     arch/arm64/crypto/aes-glue-ce.i
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>