* sysdeps/arm/sysdep.h: Define __USE_BX__ if bx is available.
Use it instead of __THUMB_INTERWORK__. Make RETINSTR take
only a condition and a register.
* sysdeps/arm/dl-machine.h: Use __USE_BX__ instead of
__THUMB_INTERWORK__.
(_dl_start_user): Use BX.
* sysdeps/arm/strlen.S: Use DO_RET.
* sysdeps/unix/arm/brk.S, sysdeps/unix/arm/fork.S,
sysdeps/unix/arm/sysdep.S, sysdeps/unix/arm/sysdep.h: Likewise.
* sysdeps/unix/sysv/linux/arm/clone.S,
sysdeps/unix/sysv/linux/arm/mmap.S,
sysdeps/unix/sysv/linux/arm/mmap64.S,
sysdeps/unix/sysv/linux/arm/socket.S,
sysdeps/unix/sysv/linux/arm/sysdep.h,
sysdeps/unix/sysv/linux/arm/vfork.S: Update uses of RETINSTR.