libdwfl: Use process_vm_readv when available.

libdwfl: Use process_vm_readv when available.

If possible use process_vm_readv to read 4K blocks instead of fetching
each word individually with ptrace. For unwinding this often means we
only have to do one process_vm_readv of the stack instead of dozens of
ptrace calls. There is one 4K cache per process, cleared whenever a
thread is detached.

Signed-off-by: Mark Wielaard <mark@klomp.org>