GNU make 4.x is needed to cross-compile glibc 2.28 and newer.
This ensures the problem is caught earlier on CentOS release 7.6, whose
default make version remains 3.82.
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.