libstdc++: Add -nostdinc++ for c++17 sources [PR100017]

libstdc++: Add -nostdinc++ for c++17 sources [PR100017]

When building a build!=host compiler, the just-built gcc can't be used
to build the target libstdc++ (because it is built for the host triplet,
not the build triplet). The top-level configure.ac sets up the build
flags for libstdc++ (and other "raw_cxx" libs) like this:

GCC_TARGET_TOOL(c++ for libstdc++, RAW_CXX_FOR_TARGET, CXX,
		[gcc/xgcc -shared-libgcc -B$$r/$(HOST_SUBDIR)/gcc -nostdinc++ -L$$r/$(TARGET_SUBDIR)/libstdc++-v3/src -L$$r/$(TARGET_SUBDIR)/libstdc++-v3/src/.libs -L$$r/$(TARGET_SUBDIR)/libstdc++-v3/libsupc++/.libs],
		c++)

The -nostdinc++ flag is only used for the IN-TREE-TOOL, i.e. when using
the just-built gcc/xgcc compiler. This means that the cross-compiler
used to build libstdc++ will add its own libstdc++ headers to the
include path. That results in the #include <cfenv> in
src/c++17/floating_to_chars.cc and src/c++17/floating_from_chars.cc
doing #include_next <fenv.h> and finding the libstdc++ fenv.h wrapper
from the host compiler. Because that has the same include guard as the
<fenv.h> in the libstdc++ we're trying to build, we never reach the
underlying <fenv.h> from libc. That results in several errors of the
form:

error: 'fenv_t' has not been declared in '::'

The most correct fix would be to add -nostdinc++ to the
RAW_CXX_FOR_TARGET variable in configure.ac, or the
RAW_CXX_TARGET_EXPORTS variable in Makefile.tpl.

Another solution would be to make the libstdc++ <fenv.h> wrapper use
_GLIBCXX_INCLUDE_NEXT_C_HEADERS like our <stdlib.h> and other C header
wrappers.

For now though, the simplest and safest solution is to just add
-nostdinc++ to the CXXFLAGS used for src/c++17/*.cc, which is what this
does.

libstdc++-v3/ChangeLog:

	PR libstdc++/100017
	* src/c++17/Makefile.am (AM_CXXFLAGS): Add -nostdinc++.
	* src/c++17/Makefile.in: Regenerate.