* Fix linker-plugin-lto: use `-flto=thin`

The linker-plugin/lld uses thin LTO by default, and it requires the archive to be compiled
with thin LTO as well, since thin LTO generates function summary
which is needed by thin LTO.

* Clarify purpose of unused 'lto' variable in flags.rs

Added a comment explaining the purpose of the unused 'lto' variable.

* Update comments on thin LTO for linker plugin

Clarify comments regarding thin LTO requirements for linker plugin.

Signed-off-by: spuradage <[email protected]>

For #1582

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

…appers (#1495)

The cross-compilation toolchains distributed by the `llvm-mingw` project
for building code for the `*-pc-windows-gnullvm` targets have a wrapper
script in place of their `clang` binaries that calls them with a
specific target, much like it has been the case with the Android NDK.
When `cc-rs` sets a explicit `--target` flag, the wrapper script breaks
down, and is no longer able to use the proper linker and libraries.

To improve on this situation and make it possible to cross-compile Rust
projects that depend on building C code with `cc-rs`, let's skip passing
this flag when cross-compilation with such a wrapper script is
attempted.

Fix typos

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