Set __OBSOLETE_MATH_DEFAULT to 0 if 'd' extension is supported (i.e.
__riscv_flen == 64).
Base on the comment for __OBSOLETE_MATH_DEFAULT:
> ... it assumes that the toolchain has ISO C99 support (hexfloat
> literals, standard fenv semantics), the target has IEEE-754 conforming
> binary32 float and binary64 double (not mixed endian) representation,
> standard SNaN representation, double and single precision arithmetics
> has similar latency and it has no legacy SVID matherr support, only
> POSIX errno and fenv exception based error handling.
Signed-off-by: Hau Hsu <hau.hsu@sifive.com>
97d483c2f8