Bug with gcvtf

[Rian Quinn]

The "gcvtf" function in newlib/newlib/libc/stdlib/efgcvt.c is in the file
before gcvt, but relies on it with an implicit definition. When attempting
to compile Newlib on x86_64 for Libc++ support, I get a compile error as
the definitions of the implicit version does not match the actual version.
To fix the issue, all I had to do was to place the gcvtf version _after_
the implementation of gcvt and it compiles fine.

Rian