BaseTools: Fix a bug of genffs command generation

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2221

The command used by multiple thread genffs feature in makefile
for testing if file exist is generated based on the toolchain family.
It should be based on the OS type.

Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

BaseTools/Source/Python/GenFds/GenFdsGlobalVariable.py

@@ -13,6 +13,7 @@ from __future__ import print_function
 from __future__ import absolute_import
 
 import Common.LongFilePathOs as os
+import sys
 from sys import stdout
 from subprocess import PIPE,Popen
 from struct import Struct
@@ -486,7 +487,7 @@ class GenFdsGlobalVariable:
 
             SaveFileOnChange(CommandFile, ' '.join(Cmd), False)
             if IsMakefile:
-                if GlobalData.gGlobalDefines.get("FAMILY") == "MSFT":
+                if sys.platform == "win32":
                     Cmd = ['if', 'exist', Input[0]] + Cmd
                 else:
                     Cmd = ['-test', '-e', Input[0], "&&"] + Cmd