Revert to UInt64 for offsetConst in SerialBridge

[abejgonzalez]

PR #1398 broke FireSim Rocket supernode builds. You can see here that the mem_host_offset is 64b wide: https://github.com/firesim/firesim/pull/1398/files#diff-098225fbdae21ed16399652d570908e98dc7918a7e98ccf118c689e41c3f0c3bL19. However, here you can see that the constructor generates a UInt32 instead of originally a 64b int: https://github.com/firesim/firesim/pull/1398/files#diff-ba1d4635003f304c25f2a454cd7882c077b1ac291d846afd39dbc65003f35f65R103. This PR fixes this issue.

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Verilog / AGFI Compatibility

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[sagark]

LGTM