Add Support For tohost/fromhost "Magic Memory"

[kathlenemagnus-mips]

Many RISC-V applications use “magic memory” to implement simple system commands in a baremetal environment. Atlas should support magic memory for ELFs that contain “tohost” and “fromhost” labels. Construction of “magic memory” should be handled when the ELF is loaded in the AtlasSystem class.

See riscv-software-src/riscv-isa-sim#364 (comment)