Port recent Android changes to mips64el

Port recent Android changes to mips64el

* exec/config-mips.m4.in (DADDI2, DADDI3): Disable at-clobbering
by assembler macros expressly.

* exec/loader-mips64el.s: Adapt from loader-mipsel.s.

* exec/configure.ac (exec_cv_as_daddi): Properly escape reg
names.

* exec/exec.c (struct exec_jump_command, exec_0): Don't define
or set `fpu_mode' if __LP64__.

* exec/exec.h (struct exec_tracee): New field `callno'.

* exec/trace.c (process_system_call): Always record the current
system call number in TRACEE lest it should be required once it
has been overwritten upon the syscall's completion.
(seccomp_system_call): Likewise.
(after_fork): Clear `tracee->callno'.

(cherry picked from commit 5bf86e2be0693c579a43759fd1da1651344d401e)