Add support RISC-V

Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>

c_src/double-conversion/utils.h

@@ -62,7 +62,8 @@
     defined(__sparc__) || defined(__sparc) || defined(__s390__) || \
     defined(__SH4__) || defined(__alpha__) || \
     defined(_MIPS_ARCH_MIPS32R2) || \
-    defined(__AARCH64EL__) || defined(__aarch64__)
+    defined(__AARCH64EL__) || defined(__aarch64__) || \
+    defined(__riscv)
 #define DOUBLE_CONVERSION_CORRECT_DOUBLE_OPERATIONS 1
 #elif defined(__mc68000__)
 #undef DOUBLE_CONVERSION_CORRECT_DOUBLE_OPERATIONS