Split ARCH_SOURCES_DIR into target and platform ones

[janvorli]

We have a single ARCH_SOURCES_DIR defined based on the platform we are building for. However, when crossbuilding tools that run on one platform, but target another one (e.g. crossgen running on x86 and generating ARM32 code), we need to use two different architecture sources dirs. One for files that are used to generate code for the target architecture and the other for files that run on the platform where the tool is executed.
That means that all .asm (.S) files belong to the second category, while some .cpp files in the target spefic folders belong to one and some to the other. For example, the excepamd64.cpp belong to the second category and jitinterfaceamd64.cpp to the first one.

[janvorli]

CC: @hseok-oh

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