gcc/4.9: add patch to enable more C++ features with uClibc

gcc/4.9: add patch to enable more C++ features with uClibc

This commit fixes bug #7250, by allowing more libstdc++ features to be
enabled with uClibc. libstdc++ wants an absolutely complete C99
support in the C library before enabling *any* feature that needs some
C99 functions. However, uClibc doesn't provide C99 complex numbers, so
libstdc++ disables a lot of C++ standard methods, even though they are
not related to C99 complex numbers.

A partial solution already existed in the patch
302-c99-snprintf.patch, but this commit replaces it by the more
complete 850-libstdcxx-uclibc-c99.patch, which is highly inspired by
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58393, except that it
doesn't rely on configure.ac checks, but simply on testing
defined(__UCLIBC__) like was done in 302-c99-snprintf.patch. This
allows to avoid having to autoreconf gcc, which is quite complicated
to achieve.

Reported-by: Richard <tarka.t.otter@gmail.com>
Cc: Richard <tarka.t.otter@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>