Use the same retain implementation for Win32 targets

Currently plc_Win32_main_retain.c is exact copy of
plc_Linux_main_retain.c, because it's not possible to use symbol
links.

It'd be nice in the future to have possibility to modify targets.code.

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                  <xsd:element name="Linux">

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                    <xsd:complexType>

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                      %(toolchain_gcc)s

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                    </xsd:complexType>

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                  </xsd:element>