Move function indirection code to nat/windows-nat

Move function indirection code to nat/windows-nat

gdb and gdbserver both look for functions in some Windows DLLs at
runtime.  This patch moves this code out of gdb and into
nat/windows-nat, so it can be shared by both programs.

gdb/ChangeLog
2021-04-30  Tom Tromey  <tromey@adacore.com>

	* windows-nat.c: Move code to nat/windows-nat.[ch].
	(_initialize_windows_nat): Call initialize_loadable.
	* nat/windows-nat.h (AdjustTokenPrivileges)
	(DebugActiveProcessStop, DebugBreakProcess)
	(DebugSetProcessKillOnExit, EnumProcessModules)
	(EnumProcessModulesEx, GetModuleInformation)
	(GetModuleFileNameExA, GetModuleFileNameExW)
	(LookupPrivilegeValueA, OpenProcessToken, GetConsoleFontSize)
	(GetCurrentConsoleFont, Wow64SuspendThread)
	(Wow64GetThreadContext, Wow64SetThreadContext)
	(Wow64GetThreadSelectorEntry): Move from windows-nat.c.
	(AdjustTokenPrivileges_ftype)
	(DebugActiveProcessStop_ftype, DebugBreakProcess_ftype)
	(DebugSetProcessKillOnExit_ftype, EnumProcessModules_ftype)
	(EnumProcessModulesEx_ftype, GetModuleInformation_ftype)
	(GetModuleFileNameExA_ftype, GetModuleFileNameExW_ftype)
	(LookupPrivilegeValueA_ftype, OpenProcessToken_ftype)
	(GetConsoleFontSize_ftype)
	(GetCurrentConsoleFont_ftype, Wow64SuspendThread_ftype)
	(Wow64GetThreadContext_ftype, Wow64SetThreadContext_ftype)
	(Wow64GetThreadSelectorEntry_ftype): Likewise.
	(initialize_loadable): Declare.
	* nat/windows-nat.c (AdjustTokenPrivileges)
	(DebugActiveProcessStop, DebugBreakProcess)
	(DebugSetProcessKillOnExit, EnumProcessModules)
	(EnumProcessModulesEx, GetModuleInformation, GetModuleFileNameExA)
	(GetModuleFileNameExW, LookupPrivilegeValueA, OpenProcessToken)
	(GetCurrentConsoleFont, GetConsoleFontSize, Wow64SuspendThread)
	(Wow64GetThreadContext, Wow64SetThreadContext)
	(Wow64GetThreadSelectorEntry): Define.
	(bad, bad_GetCurrentConsoleFont, bad_GetConsoleFontSize): Move
	from windows-nat.c.
	(initialize_loadable): Likewise, and rename.