x86: Add sysdeps/x86/ldsodefs.h

Merge sysdeps/i386/ldsodefs.h and sysdeps/x86_64/ldsodefs.h into
sysdeps/x86/ldsodefs.h.

Tested on i686 and x86-64.

	* sysdeps/i386/ldsodefs.h: Removed.
	* sysdeps/x86_64/ldsodefs.h: Moved to ...
	* sysdeps/x86/ldsodefs.h: This.
	(La_i86_regs): New.
	(La_i86_retval): Likewise.
	(ARCH_PLTENTER_MEMBERS): Add i86_gnu_pltenter.
	(ARCH_PLTEXIT_MEMBERS): i86_gnu_pltexit.

Acked-by: Christian Brauner (Ubuntu) <christian@brauner.io>