[RS6000] Use config/linux.h for powerpc*-*-linux*

[RS6000] Use config/linux.h for powerpc*-*-linux*

Using the macros in config/linux.h rather than duplicating them helps
stop future bitrot, and repairs existing bitrot (4 choices for libc in
linux.h, fewer in the rs6000 files not that it matters much).  Also
fixes the fact that __gnu_linux__ was always defined rather than just
when glibc was the libc of choice.

	* config.gcc (powerpc*-*-linux*): Add linux.h to tm_file.
	* config/rs6000/linux.h (TARGET_OS_CPP_BUILTINS): Use
	GNU_USER_TARGET_OS_CPP_BUILTINS.
	(RS6000_ABI_NAME): Define.
	* config/rs6000/linux64.h (TARGET_OS_CPP_BUILTINS): Use
	GNU_USER_TARGET_OS_CPP_BUILTINS.
	(MUSL_DYNAMIC_LINKER32): Undef before defining.
	(UCLIBC_DYNAMIC_LINKER32, UCLIBC_DYNAMIC_LINKER64): Don't define.
	(CHOOSE_DYNAMIC_LINKER): Don't define.
	(GNU_USER_DYNAMIC_LINKER32, GNU_USER_DYNAMIC_LINKER64): Don't define.
	* config/rs6000/sysv4.h (MUSL_DYNAMIC_LINKER): Undef before defining.
	(CHOOSE_DYNAMIC_LINKER, GNU_USER_DYNAMIC_LINKER): Only define when
	not already defined.
	(CPP_OS_LINUX_SPEC): Remove defines and asserts handled by
	TARGET_OS_CPP_BUILTINS.

From-SVN: r266493