Use GetSystemTimeAsFileTime directly in win32 gettimeofday

PostgreSQL was calling GetSystemTime followed by SystemTimeToFileTime in the
win32 port gettimeofday function. This is not necessary and limits the reported
precision to the 1ms granularity that the SYSTEMTIME struct can represent. By
using GetSystemTimeAsFileTime we avoid unnecessary conversions and capture
timestamps at 100ns granularity, which is then rounded to 1µs granularity for
storage in a PostgreSQL timestamp.

On most Windows systems this change will actually have no significant effect on
timestamp resolution as the system timer tick is typically between 1ms and 15ms
depending on what timer resolution currently running applications have
requested. You can check this with clockres.exe from sysinternals. Despite the
platform limiation this change still permits capture of finer timestamps where
the system is capable of producing them and it gets rid of an unnecessary
syscall.

The higher resolution GetSystemTimePreciseAsFileTime call available on Windows
8 and Windows Server 2012 has the same interface as GetSystemTimeAsFileTime, so
switching to GetSystemTimeAsFileTime makes it easier to use the Precise variant
later.
On Windows, use GetSystemTimePreciseAsFileTime when available

PostgreSQL on Windows 8 or Windows Server 2012 will now obtain
high-resolution timestamps by dynamically loading the the
GetSystemTimePreciseAsFileTime function. It'll fall back to
GetSystemTimeAsFileTime if the higher precision variant isn't found,
so the same binaries without problems on older Windows releases.

No attempt is made to detect the Windows version. Only the presence or
absence of the desired function is considered.