float_common.h: Support RISC-V

Author: leiwen2025

include/fast_float/float_common.h

@@ -93,11 +93,13 @@ using parse_options = parse_options_t<char>;
      defined(__MINGW64__) || defined(__s390x__) ||                             \
      (defined(__ppc64__) || defined(__PPC64__) || defined(__ppc64le__) ||      \
       defined(__PPC64LE__)) ||                                                 \
-     defined(__loongarch64))
+     defined(__loongarch64) ||                                                 \
+     (defined(__riscv) && __riscv_xlen == 64))
 #define FASTFLOAT_64BIT 1
 #elif (defined(__i386) || defined(__i386__) || defined(_M_IX86) ||             \
        defined(__arm__) || defined(_M_ARM) || defined(__ppc__) ||              \
-       defined(__MINGW32__) || defined(__EMSCRIPTEN__))
+       defined(__MINGW32__) || defined(__EMSCRIPTEN__) ||                      \
+       (defined(__riscv) && __riscv_xlen == 32))
 #define FASTFLOAT_32BIT 1
 #else
   // Need to check incrementally, since SIZE_MAX is a size_t, avoid overflow.