arm64: vdso: Compile vDSO at O3 with Clang

Subsequent to c251c5f ("Makefile: Use O3 for Clang")

Ensure vDSO uses the same optimization level as the rest of the
kernel, which is O3, when compiling with Clang.

Signed-off-by: Adam W. Willis <return.of.octobot@gmail.com>
Signed-off-by: Adithya R <gh0strider.2k18.reborn@gmail.com>
Signed-off-by: iusmac <iusico.maxim@libero.it>
Signed-off-by: improbir <ghosh_probir@yahoo.com>