doc: Fix documentation around 'asm' keyword in C++

doc: Fix documentation around 'asm' keyword in C++

The documentation on asm statements suggests asm is always a GNU
extension, but it's been part of ISO C++ since the first standard.

The documentation of -fno-asm is wrong for C++ as it states that it only
affects typeof, but actually it affects typeof and asm (despite asm
being part of ISO C++).

gcc/ChangeLog:

	* doc/extend.texi (Basic Asm): Clarify that asm is not an
	extension in C++.
	* doc/invoke.texi (-fno-asm): Fix description for C++.