aarch64: Move ld.so _start to separate file and drop _dl_skip_args

aarch64: Move ld.so _start to separate file and drop _dl_skip_args

A separate asm file is easier to maintain than a macro that expands to
inline asm.

The RTLD_START macro is only needed now because _dl_start is local in
rtld.c, but _start has to call it, if _dl_start was made hidden then it
could be empty.

_dl_skip_args is no longer needed.

---
v4:
- adjust commit message about _dl_skip_args.
v3:
- mention _dl_skip_args
v2:
- fix typo in commit message.