runtime: always use LoadLibraryEx to load system libraries

runtime: always use LoadLibraryEx to load system libraries

This CL removes a fallback that used LoadLibraryA when the runtime
was loading system DLLs on Windows 7, Windows Server 2008 R2,
or earlier.

We can safely remove that fallback now, as go1.21 will require at least
Windows 8 or Server 2012.

This CL also saves some syscall initialization time and bytes:

new:
    init syscall @2.3 ms, 0 ms clock, 1000 bytes, 18 allocs
old:
    init syscall @3.6 ms, 0.52 ms clock, 1744 bytes, 24 allocs

Updates #57003

Change-Id: I7dcc1173537785b6b580e9f78632c0c74da658d4
Reviewed-on: https://go-review.googlesource.com/c/go/+/463842
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>