x32: Implement prctl in assembly

x32: Implement prctl in assembly

Since the variadic prctl function takes at most 5 integer arguments which
are passed in the same integer registers on x32 as the function with 5
integer arguments, we can use assembly for prctl.  Since upper 32-bits in
the last 4 arguments of pcrtl must be cleared to match the x32 prctl
syscall interface where the last 4 arguments are unsigned 64 bit longs,
implement prctl in assembly to clear upper 32-bits in the last 4 arguments
and add a test to verify it.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Florian Weimer <fweimer@redhat.com>