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[fortran,patch] Patch for PR 14394 - Precision of real type.

Hi, all
  This patch fix PR 14394. Reported by Bud. Test case:
cat z.f
        REAL AVS,BVS
        BVS = 1.5747025767
        AVS = TAN(BVS)
  There is one problem converting the real type from mpf to gcc tree. With
-fdump-parse-tree we can see fortran gets the input and parse correctly.
gfc_conv_mpf_to_tree in trans-const.c use string as the intermediate of the
conversion. As convert mpf to string, we specify the maximum digits number of
the real value. In my patch, I loosen the limit. 
  I tested real and real*8 type with the patch. The real type got the same
result with Intel's compiler. But with real*8 type, gfortran gave the result:
-255.998704134517, compared with Intel's -255.995833263172. The difference of
them is above 0.002. With calculator I comfirmed the gfortran's result should
be correct. But can anyone give more comparation with other compilers. What's
your opinion?

Feng Wang

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