Bump glibc and linux kernel

 - RV32 require 5.10 which is too new for most linux distro.

Bump gdb for ctf related build fix.

Use `git rev-parse` to find .git dir

Otherwise, this code breaks if riscv-gnu-toolchain is itself a
submodule of another project. In that case the git config for this
tree will be $(srcdir)/../.git/modules/riscv-gnu-toolchain/config.

Automatic Nightly Release

Add new precompile process for rv32/64-elf toolchain: @higuoxing

1. repository activity check
2. change to build.yaml
3. assets layout
4. matrix action build
5. run daily too & build on PR
6. upload all assets to release
7. Only run nightly

[CI] Improve GitHub Action scripts.

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.

Add doc for development

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