sim: Add a "make sim" target

This allows users without hardware to simulate Linux on Spike.

.gitmodules

@@ -10,3 +10,9 @@
 [submodule "riscv-gnu-toolchain"]
 	path = riscv-gnu-toolchain
 	url = https://github.com/riscv/riscv-gnu-toolchain.git
+[submodule "riscv-isa-sim"]
+	path = riscv-isa-sim
+	url = https://github.com/riscv/riscv-isa-sim.git
+[submodule "riscv-fesvr"]
+	path = riscv-fesvr
+	url = https://github.com/riscv/riscv-fesvr.git