[RISC-V] Update TODOs for RiscV64 Machine and Architecture in Crossgen2

[gbalykov]

Currently in RISC-V Crossgen2 port (Architecture)9 and (Machine)0x5064 are used instead of Architecture.RiscV64 and Machine.RiscV64 correspondingly, because those are not yet propagated to .NET SDK. Update them when new .NET SDK is shipped.

Related PR with initial RISC-V support in Crossgen2: #95188.

cc @clamp03 @ashaurtaev

[am11]

Blocked on #95980.