[RS6000] PR91135, __linux__ not defined with -mcall-aixdesc on 9.x and ppc64

[RS6000] PR91135, __linux__ not defined with -mcall-aixdesc on 9.x and ppc64

This patch makes the obvious fix for PR91135, and deletes extraneous
copies of GNU_USER_TARGET_D_OS_VERSIONS that appear in rs6000/linux.h
and rs6000/linux64.h.  Since all configurations using either of these
files also include linux.h there is no need to duplicate the macro.

	PR target/91135
	* config/rs6000/linux.h (GNU_USER_TARGET_D_OS_VERSIONS): Don't
	define.
	* config/rs6000/linux64.h (TARGET_OS_CPP_BUILTINS): Invoke
	GNU_USER_TARGET_OS_CPP_BUILTINS for aixdesc abi.
	(GNU_USER_TARGET_D_OS_VERSIONS): Don't define.

From-SVN: r273854