* Replace SiFive `riscv64-unknown-elf-elf2hex` utility with custom script

`riscv64-unknown-elf-elf2hex` is a frequent barrier to easily using Wally on a system that has not gone through our full installation process. Without this tool, the only thing that *needs* to be installed to run an ELF on Wally is a simulator (Verilator, Questa, etc) and a compiler. This makes it much easier to fit Wally into other flows. I've run into this a few times over the last year where `riscv64-unknown-elf-elf2hex` in particular caused issues.

* Update shebang