Linux: Switch back to assembly syscall wrapper for prctl (bug 29770)

Commit ff026950 ("Add a C wrapper for
prctl [BZ #25896]") replaced the assembler wrapper with a C function.
However, on powerpc64le-linux-gnu, the C variadic function
implementation requires extra work in the caller to set up the
parameter save area.  Calling a function that needs a parameter save
area without one (because the prototype used indicates the function is
not variadic) corrupts the caller's stack.   The Linux manual pages
project documents prctl as a non-variadic function.  This has resulted
in various projects over the years using non-variadic prototypes,
including the sanitizer libraries in LLVm and GCC (GCC PR 113728).

This commit switches back to the assembler implementation on most
targets and only keeps the C implementation for x86-64 x32.

Also add the __prctl_time64 alias from commit
b39ffab8 ("Linux: Add time64 alias for
prctl") to sysdeps/unix/sysv/linux/syscalls.list; it was not yet
present in commit ff026950.

This restores the old ABI on powerpc64le-linux-gnu, thus fixing
bug 29770.

Reviewed-By: Simon Chopin <simon.chopin@canonical.com>