selftests/vDSO: remove duplicate compiler invocations from Makefile

The Makefile open-codes compiler invocations that ../lib.mk already
provides.

Avoid this by using a Make feature that allows setting per-target
variables, which in this case are: CFLAGS and LDFLAGS. This approach
generates the exact same compiler invocations as before, but removes all
of the code duplication, along with the quirky mangled variable names.
So now the Makefile is smaller, less unusual, and easier to read.

The new dependencies are listed after including lib.mk, in order to
let lib.mk provide the first target ("all:"), and are grouped together
with their respective source file dependencies, for visual clarity.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>