raw_stream formatter: [Win32] Use std::signbit() if available, instead of _fpclass().
raw_stream formatter: [Win32] Use std::signbit() if available, instead of _fpclass().

FIXME: It should be generic to C++11. For now, it is dedicated to mingw-w64.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@199052 91177308-0d34-0410-b5e6-96231b3b80d8

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