mesa: use C locale for _mesa_strtod()

mesa: use C locale for _mesa_strtod()

_mesa_strtod() is used for shader/program parsing where the decimal
point character is always '.'  Use strtod_l() with a "C" locale to
ensure correct string->double conversion when the actual locale uses
another character such as ',' for the decimal point.

Fixes bug 24531.