BaseTools: Fix build failure when multiple build targets given

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2371

This patch is to fix a regression issue that build fails
if multiple build targets given.

Two changes cause this regression issue.
One is AutoGen object __hash__ function only
hash file path and arch, missing ToolChain and build target.

The other is changing the multiple-thread-genfds function as default
build behavior. To generate the genffs command to Makefile, there
is a global data set is used, GenFdsGlobalVariable, which cause build
tool use the data of first build-target build in
the second build-target build.

Signed-off-by: Bob Feng <bob.c.feng@intel.com>

Cc: Liming Gao <liming.gao@intel.com>
Cc: Steven Shi <steven.shi@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>

BaseTools/Source/Python/AutoGen/PlatformAutoGen.py

@@ -148,7 +148,7 @@ class PlatformAutoGen(AutoGen):
     #
     @cached_class_function
     def __hash__(self):
-        return hash((self.MetaFile, self.Arch))
+        return hash((self.MetaFile, self.Arch,self.ToolChain,self.BuildTarget))
     @cached_class_function
     def __repr__(self):
         return "%s [%s]" % (self.MetaFile, self.Arch)