toolchains: Flatten riscv-tools submodule

This allows individual components to be better maintained following the
deprecation of riscv-tools.  Eliminate non-essential submodules.

build-static-libfesvr.sh is no longer necessary since libfesvr.a is
built as part of the riscv-isa-sim build.

For simplicity, only riscv-gnu-toolchain is now pre-built instead of the
entirety of riscv-tools.