libstdc++: Fix C++ 11 ctype when using picolibc (blank vs space)

libstdc++: Fix C++ 11 ctype when using picolibc (blank vs space)

Existing toolchain builds rely on the similarity between picolibc and
newlib when building libstdc++ and use --with-newlib.

Switch to the picolibc 16-bit _ctype_wide array which provides
separate values for ctype_base::blank and ctype_base::space.

This fixes a bug where libstdc++ was including '\f', '\n', '\r' and
'\v' in the set of 'blank' chars. Afterwards, only ' ' and '\t' are in
this set, as specified by C++ 11.

libstdc++-v3/ChangeLog:

	* acinclude.m4 (GLIBCXX_CONFIGURE): Add --with-picolibc.
	* configure: Regenerate.
	* configure.ac: Add handling for with_picolibc=yes.
	* config/os/picolibc/ctype_base.h: New file.
	* config/os/picolibc/ctype_configure_char.cc: New file.
	* config/os/picolibc/ctype_inline.h: New file.
	* config/os/picolibc/os_defines.h: New file.

Signed-off-by: Keith Packard <keithp@keithp.com>