Port recent changes to mipsel systems

Port recent changes to mipsel systems

* exec/exec.c (exec_0): Don't disable AT_EXECFN substitution on
MIPS systems.

* exec/loader-aarch64.s (skip_environ): Correct typo in
commentary.

* exec/loader-mips64el.s: Add a disclaimer that this file is
currently inoperable.

* exec/loader-mipsel.s (__start): Move environment and argument
vectors and produce and replace AT_EXECFN.  Clear stack before
proceeding to circumvent an oversight in glibc.

(cherry picked from commit 74df372398dbc90f6c0185f1701af28129073de7)