Split the runtime arch property structs into 3

Instead of creating a runtime generated struct type:
struct {
    Arch struct {
        Arm struct {
            Armv7-a struct {
                ...
            }
        }
        ...
    }
    Multilib struct {
        Lib32 struct {
            ...
        }
        ...
    }
    Target struct {
        Android struct {
            ...
        }
        ...
    }
}

Replace it with a top-level struct of empty interfaces:
type archPropRoot struct {
        Arch, Multilib, Target interface{}
}

And then embed nil pointers to smaller structs inside it:
struct {
    Arm struct {
        Armv7-a struct {
            ...
        }
    }
    ...
}

This reduces the maximum number of times the archecture specific
properties are embedded in a runtime generated struct from 139
to 97, which makes it easier to avoid hitting the runtime generated
struct name size limit of 64kB.

Bug: 146234651
Test: all soong tests
Change-Id: I901c434ac9a1d99c74fc2370c5e6803b3682dc73