Update Android port

Update Android port

* Makefile.in (extraclean): Clean in exec as well.
* configure.ac: Fix detection of absolute srcdir.  Also, pass
CFLAGS.
* exec/Makefile.in: (.c.o): Add -I. so config.h can be
found.:(.s.o): Don't create m4 temporary in srcdir.
* exec/config-mips.m4.in (DADDI2, DADDI3): New macros.  Define
to substitute if as cannot assemble daddi.
* exec/configure.ac (user_h): Look for user.h in asm/ as well.
Use new user.h.  Also look in ptrace.h on arm systems.  Check if
as supports daddi on mips64.
* exec/exec.c (check_interpreter): Fix char signedness bug.
* exec/loader-mips64el.s (__start): Use DADDI2 and DADDI3 for
two- and 3-operand daddi.
* exec/mipsel-user.h: Don't include sgidefs.h.
* java/INSTALL: Document that m4 is now required.
* src/android.c (android_rewrite_spawn_argv): Add missing NULL.