linux(4): Validate exec_setregs on arm64

The lr register is cleared at the beginning of the _dl_start and _start,
so there is no need to initialize it.
Gnu libc _start takes an rtld_fini pointer in x0 which is set by ld.so
for __libc_start_main, the kernel does not register any atexit pointers.

While here fix whitespaces.

MFC after: 1 week