[Driver] Make -static-libgcc imply static libunwind

In the GNU toolchain, `-static-libgcc` implies that the unwindlib will
be linked statically. However, when `--unwindlib=libunwind`, this flag is
ignored, and a bare `-lunwind` is added to the linker args.  Unfortunately,
this means that if both `libunwind.so`, and `libunwind.a` are present
in the library path, `libunwind.so` will be chosen in all cases where
`-static` is not set.

This change makes `-static-libgcc` affect the `-l` flag produced by
`--unwindlib=libunwind`. After this patch, providing
`-static-libgcc --unwindlib=libunwind` will cause the driver to explicitly
emit `-l:libunwind.a` to statically link libunwind. For all other cases
it will emit `-l:libunwind.so` matching current behavior with a more
explicit link line.

https://reviews.llvm.org/D70416

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