BaseTools:Fixed build failure when specifying multiple BUILDTARGET

With Python3, the dict.value() method returns an iterator.
If a dictionary is updated while an iterator on its keys is used,
a RuntimeError is generated.
Converting the iterator to a list() forces a copy of the mutable
keys in an immutable list which can be safely iterated.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Zhiju.Fan <zhijux.fan@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>

BaseTools/Source/Python/AutoGen/AutoGen.py

@@ -580,7 +580,7 @@ class WorkspaceAutoGen(AutoGen):
                                 if NewPcd2 not in GlobalData.MixedPcd[item]:
                                     GlobalData.MixedPcd[item].append(NewPcd2)
 
-            for BuildData in PGen.BuildDatabase._CACHE_.values():
+            for BuildData in list(PGen.BuildDatabase._CACHE_.values()):
                 if BuildData.Arch != Arch:
                     continue
                 for key in BuildData.Pcds: