linux(4): Make vDSO defines private.

Hide the vDSO defines to the linux32_sysvec as they are not intended to
be used outside of it. Fix LINUX32_PS_STRINGS, use the size of
struct linux32_ps_strings instead of a numeric constant.

MFC after: 2 weeks

(cherry picked from commit 4aae133469c6bc4ace55bc54eee552971e37e1c4)