This patch helps improve GitHub Action scripts.
 * Make CI happy again after d3dfbee6.
 * Simplify the logic of GH Action scripts using 'matrix'.
 * Add new precompile process for rv32/64-elf toolchain.

Automatically download sub-module on-demand and add branch info.

 - It's too new to use, that require git 2.26+

 - So that we can build and clone in parallel.

Update README.md for clear instructions on multilib

Fix testing when configure with --with-multilib-generator=

 - Current script will only handle first 2 items in
   `--with-multilib-generator=`.

The current README.md only explains how to use multilib on the Linux cross-compiler, but not on the Newlib one. This might create confusion as seen here https://github.com/riscv/riscv-tools/issues/287, or makes it seem like you can't multilib on Newlib. It's also the case that the multilib option is more common on embedded toolchains rather than on Linux, so I believe this small change on documentation should clear things up.

Bump and switch to using upstream repo for newlib

Replace `build-essential` with `base-devel` in the installation guide for Arch Linux.

Disable libsanitizer when building the musl-based toolchain

This allows building the musl-based toolchain again, tested
with the latest stable version of musl-libc.

Fix simulator dependency for spike

Fix always re-run testsuite issue

 - This issue is report by Nelson.

 - Don't using build-sim as dependency, it will cause the Make always
   rebuild.

quote variable so that semicolons are not interpreted by the shell

fix spelling

Fix last gcc bump.

Commit ids changed on branch merge, so accidentally pointing at wrong
commit id, on deleted branch.

Bump gdb to FSF gdb 10.1, with our gdb sim patched in.

Bump gcc and add option to configure multi-lib

Backport following patches from upstream:

 - RISC-V: Check multiletter extension has more than 1 letter
 - RISC-V: Add configure option: --with-multilib-generator to flexible config multi-lib settings.
 - RISC-V: Refine riscv_parse_arch_string
 - RISC-V: Extend syntax for the multilib-generator
 - RISC-V: Handle implied extension in multilib-generator
 - RISC-V: Add support for -mcpu option.
 - RISC-V: Define __riscv_cmodel_medany for PIC mode.

Support spike as simulator to run gcc testsuite

Vagrant plugin check so it will not fail in the middle of provisioning

Bump gcc for zstd fix.