riscv: s/x3/gp and s/x4/tp in seL4_UserContext

This makes seL4_UserContext consistent with frameRegisters

riscv: seL4_RISCV_VMAttributes matches vm_attributes_t

Updates the user visible seL4_RISCV_VMAttributes definitions to match what is declared
in vm_attributes_t, which is just the executeNever attribute.

RISCV: Place TODOs in the source

[SELFOUR-1156] RISC-V Port

Experimental release that supports both RV32 and RV64