* CI: Create regenerate-target-info

For automatically regenerate target_info.rs

* Update step for generating branch name in regenerate-target-info

* Fix regenerate-target-info.yml: install nightly rust toolchain

* Fix branch created in regenerate-target-info.yml

Make it less confusing for gh

* Add rust-cache to regenerate-target-info.yml

* Fix creating PR in regenerate-target-info.yml

* Fix creating PR in regenerate-target-info.yml

Pass `--body` to `gh pr create`

* Fix title of PR generated by regenerate-target-info.yml

Co-authored-by: github-actions <github-actions@github.com>

* Don't use the value of SDKROOT if it doesn't match the target.

The SDKROOT environment variable can be problematic on Mac.
In cases where you are compiling for multiple targets, SDKROOT
cannot be right for all of them. Furthermore, the system Python
interpreter sets SDKROOT to the MacOSX SDK if not already set.

So, if you are using a Python script to run a build, and you need
to build for multiple targets, the logic in the cc crate doesn't work.
This is precisely what is happening with rustc itself, and so we can't
upgrade the version of the cc crate used by the bootstrap code.

(Unsetting SDKROOT doesn't work either because the custom clang
build that rustc uses for CI depends on it being set)

* use env::var_os instead of env::var

* Fix compilation

* Fix compilation error

* fix compilation

---------

Co-authored-by: Jiahao XU <Jiahao_XU@outlook.com>

when no default flag is specificed

…he (#1050)