Fix program execution on Android 15 QPR2 Beta

Fix program execution on Android 15 QPR2 Beta

* exec/trace.c (process_vm_readv, process_vm_writev): New
function pointers.  Attempt to load them on recent Android
systems when `exec' was not linked with a sufficiently
up-to-date libc.
(read_memory, user_copy): Always use process_vm_readv and
process_vm_writev if available.
(handle_openat): Write trailing NULL byte of filename to user
buffer.
(exec_init): Attempt to dlsym process_vm_readv and
process_vm_writev.