Enable shallow git submodule (#350)

Author: howjmay

.gitmodules

@@ -1,10 +1,13 @@
 [submodule "riscv-arch-test"]
 	path = tests/riscv-arch-test
 	url = https://github.com/riscv-non-isa/riscv-arch-test
+	shallow = true
 [submodule "mini-gdbstub"]
 	path = src/mini-gdbstub
 	url = https://github.com/RinHizakura/mini-gdbstub
+	shallow = true
 [submodule "softfloat"]
 	path = src/softfloat
 	url = https://github.com/sysprog21/berkeley-softfloat-3
 	branch = rv32emu
+	shallow = true