Replace NO_IMPLICIT_EXTERN_C with SYSTEM_IMPLICIT_EXTERN_C.

Replace NO_IMPLICIT_EXTERN_C with SYSTEM_IMPLICIT_EXTERN_C.

	gcc/
	Replace NO_IMPLICIT_EXTERN_C with SYSTEM_IMPLICIT_EXTERN_C.
	* doc/cpp.texi: Update comment.
	* doc/tm.texi: Rebuilt.
	* doc/tm.texi.in (NO_IMPLICIT_EXTERN_C): Replace with ...
	(SYSTEM_IMPLICIT_EXTERN_C): ... this, opposite sense.
	* doc/extend.texi (Backwards Compatibility): Clarify it is system
	headers affected by extern "C".
	* system.h: Poison NO_IMPLICIT_EXTERN_C.
	* config/alpha/alpha.h, config/arm/uclinux-elf.h,
	config/bfin/elf.h, config/cris/cris.h, config/darwin.h,
	config/dragonfly.h, config/freebsd.h, config/gnu-user.h,
	config/i386/cygming.h, config/i386/djgpp.h, config/i386/nto.h,
	config/ia64/hpux.h, config/lm32/lm32.h, config/lm32/uclinux-elf.h,
	config/lynx.h, config/mips/elf.h, config/mmix/mmix.h,
	config/netbsd.h, config/pa/pa-hpux.h, config/powerpcspe/sysv4.h,
	config/riscv/elf.h, config/rs6000/sysv4.h, config/rtems.h,
	config/s390/tpf.h, config/sh/newlib.h, config/sol2.h,
	config/sparc/openbsd64.h, config/sparc/sp-elf.h,
	config/sparc/sp64-elf.h, config/spu/spu.h,
	config/stormy16/stormy16.h, config/v850/v850.h,
	config/visium/visium.h, config/vx-common.h, config/xtensa/elf.h: Don't
	define NO_IMPLICIT_EXTERN_C.
	* config/rs6000/aix.h: Set SYSTEM_IMPLICIT_EXTERN_C.

	gcc/c-family/
	* c-lex.c (fe_file_change): Check SYSTEM_IMPLICIT_EXTERN_C not
	NO_IMPLICIT_EXTERN_C.

	gcc/cp/
	* cp/decl.c (decls_match): Check SYSTEM_IMPLICIT_EXTERN_C not
	NO_IMPLICIT_EXTERN_C.
	* cp/parser.c (cp_parser_parameter_declaration_clause): Likewise.

From-SVN: r262437