RISC-V: Use the new libm code if possible

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>

newlib/libc/include/machine/ieeefp.h

@@ -221,6 +221,11 @@
 #ifdef __riscv_flen
 # define _SUPPORTS_ERREXCEPT
 #endif
+#if __riscv_flen == 64
+# define __OBSOLETE_MATH_DEFAULT 0
+#else
+# define __OBSOLETE_MATH_DEFAULT 1
+#endif
 #endif
 
 #ifdef __i960__