Auto merge of #37822 - cuviper:llvm-link-shared, r=alexcrichton

rustbuild: allow dynamically linking LLVM

The makefiles and `mklldeps.py` called `llvm-config --shared-mode` to
find out if LLVM defaulted to shared or static libraries, and just went
with that.  But under rustbuild, `librustc_llvm/build.rs` was assuming
that LLVM should be static, and even forcing `--link-static` for 3.9+.

Now that build script also uses `--shared-mode` to learn the default,
which should work better for pre-3.9 configured for dynamic linking, as
it wasn't possible back then to choose differently via `llvm-config`.

Further, the configure script now has a new `--enable-llvm-link-shared`
option, which allows one to manually override `--link-shared` on 3.9+
instead of forcing static.

Update: There are now four static/shared scenarios that can happen
for the supported LLVM versions:

- 3.9+: By default use `llvm-config --link-static`
- 3.9+ and `--enable-llvm-link-shared`: Use `--link-shared` instead.
- 3.8: Use `llvm-config --shared-mode` and go with its answer.
- 3.7: Just assume static, maintaining the status quo.

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