util: port _mesa_strto[df] to C

util: port _mesa_strto[df] to C

_mesa_strtod and _mesa_strtof are only used from the GLSL compiler and
the ARB_[vertex|fragment]_program code, meaning that the locale doesn't
need to be initialized before the first OpenGL context gets initialized.

So let's use explicit initialization from the one-time init code instead
of depending on a C++ compiler to initialize at image-load time.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>