Use strtod instead of strtold in libiberty/d-demangle.c

Use strtod instead of strtold in libiberty/d-demangle.c

strtold is currently used to decode templates which have a floating-point
value encoded inside; but this routine is not available on some systems,
such as Solaris 2.9 for instance.

This patch fixes the issue by replace the use of strtold by strtod.
It reduces a bit the precision, but it should still remain acceptable
in most cases.

libiberty/ChangeLog:

        * d-demangle.c: Replace strtold with strtod in global comment.
        (strtold): Remove declaration.
        (strtod): New declaration.
        (dlang_parse_real): Declare value as double instead of long
        double.  Replace call to strtold by call to strtod.
        Update format in call to snprintf.