[compiler-rt] Fix a warning

This patch fixes:

  compiler-rt/lib/builtins/int_to_fp_impl.inc:22:18: error: expression
  is not an integer constant expression; folding it to a constant is a
  GNU extension [-Werror,-Wgnu-folding-constant]

by using enum for constants.

Author: kazutakahirata

compiler-rt/lib/builtins/int_to_fp.h

@@ -53,13 +53,19 @@ enum {
 typedef double dst_t;
 typedef uint64_t dst_rep_t;
 #define DST_REP_C UINT64_C
-static const int dstSigBits = 52;
+
+enum {
+  dstSigBits = 52,
+};
 
 #elif defined DST_QUAD
 typedef long double dst_t;
 typedef __uint128_t dst_rep_t;
 #define DST_REP_C (__uint128_t)
-static const int dstSigBits = 112;
+
+enum {
+  dstSigBits = 112,
+};
 
 #else
 #error Destination should be a handled floating point type