Build newlib with POSIX mode

 - newlib was default to XOPEN MODE before 2018/12/6[1], but newlib
   decide to remove XOPEN mode SVID3 mode, and change the default IEEE
   mode.

 - The problem is IEEE won't set errno on several math function, so
   build with POSIX mode would be better for newlib.

 - For newlib nano, we might able to build with _IEEE_LIBM in future
   to reduce code size, and improve performance.

[1] https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;a=commit;h=b14a879d85b171960df789ac8ba2332004f838e0