BaseTools: Fix build failure when specifying multiple BUILDTARGET

BaseTools: Fix 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.

Commit f8d11e5a4aaa converted various uses but missed one:
When specifying multiple BUILDTARGET, the first target builds
successfully, but then the PGen.BuildDatabase._CACHE_ dictionary is
updated, and accessing the next target triggers a RuntimeError.

Convert this iterator to an immutable list, to solve this build error:

    $ build -a IA32 -t GCC5 -b RELEASE -b NOOPT -p OvmfPkg/OvmfPkgIa32.dsc
    [...]
    Processing meta-data ...
    build.py...
     : error C0DE: Unknown fatal error when processing [OvmfPkg/OvmfPkgIa32.dsc]

    (Please send email to edk2-devel@lists.01.org for help, attaching following call stack trace!)

    (Python 3.5.3 on linux) Traceback (most recent call last):
      File "BaseTools/BinWrappers/PosixLike/../../Source/Python/build/build.py", line 2387, in Main
        MyBuild.Launch()
      File "BaseTools/BinWrappers/PosixLike/../../Source/Python/build/build.py", line 2141, in Launch
        self._MultiThreadBuildPlatform()
      File "BaseTools/BinWrappers/PosixLike/../../Source/Python/build/build.py", line 1921, in _MultiThreadBuildPlatform
        self.Progress
      File "BaseTools/Source/Python/AutoGen/AutoGen.py", line 304, in __init__
        self._InitWorker(Workspace, MetaFile, Target, Toolchain, Arch, *args, **kwargs)
      File "BaseTools/Source/Python/AutoGen/AutoGen.py", line 477, in _InitWorker
        for BuildData in PGen.BuildDatabase._CACHE_.values():
    RuntimeError: dictionary changed size during iteration

Note: The culprit commit (f8d11e5a4aaa) can not be found with bisection.
In 9c2d68c0a299 the build tools default to the python version provided
by the ${PYTHON} environment variable, however the Python3 transition is
not functional before d943b0c339fe. f8d11e5a4aaa falls between the
previous two.

Reported-by: Leif Lindholm <leif.lindholm@linaro.org>
Fixes: f8d11e5a4aaa90bf63b4789f3993dd6d16c60787
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Philippe Mathieu-Daude <philmd@redhat.com>
Tested-by: Leif Lindholm <leif.lindholm@linaro.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>