Accpet OsStr instead of str for flags (#1100)

* Update `getenv` to return `Arc<Ostr>` instead of `Arc<str>`

It is possible that the environment variable contains non-utf8 charactes

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

* Update `get_cpp_link_stdlib` to return `Result<Option<Cow<'_, Path>>, Error>`

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

* Update `cpp_{set, link}_stdlib` to accept `Path` instead of `str`

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

* Optimize `prefix_for_target` to ret `Cow<'static, str>`

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

* Accept `impl AsRef<OsStr>` for asm, ar and cc/cxx flags and `link_lib_modifiers`

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

* Improve debug logging in `getenv`

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

* Revert deprecation

* Revert `cpp_{set, link}_stdlib` changes

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

* Fix doc

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

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Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>