On macOS, "gnumake" and "make" are both supplied by the OS, but are too
old to build glibc (both are version 3.81 as of this writing). Homebrew
provides the "gmake" executable, which is recent enough for glibc.
However, the existing logic in "scripts/build-util.sh" will always
prefer "gnumake" over "gmake".
The configure logic in the riscv-glibc library allows a user to override
the preference for "gnumake" by setting the MAKE environment variable.
This change makes "scripts/build-openocd.sh" and
"scripts/build-toolchains.sh" mimic that behavior. A user can now use
"gmake" instead of "gnumake" during the toolchain build like so:
MAKE=gmake ./scripts/build-toolchains.sh
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.
