selftests: vdso: Use a header file to prototype parse_vdso API

Both vdso_test_gettimeofday and vdso_standalone_test_x86 use the library in
parse_vdso.c but each separately declares the API it offers which is not
ideal. Create a header file with prototypes of the functions and use it in
both the library and the tests to ensure that the same prototypes are used
throughout.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

selftests: move vDSO tests from Documentation/vDSO

Remove vDSO from Makefile to move the to selftests. Update vDSO Makefile
to work under selftests. vDSO will not be run as part of selftests suite
and will not be included in install targets. They can be built separately
for now.

Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>