Fix import lib generation under MinGW

[kleisauke]

Override import_name after parsing the .def file, as -Wl,--output-def, in LLVM and Binutils does not include a library name in the generated .def file.

cargo-c/src/target.rs

Lines 112 to 118 in c37aec4

	} else if os == "windows" && env == "gnu" {
	// This is only set up to work on GNU toolchain versions of Rust
	lines.push(format!(
	"-Wl,--output-def,{}",
	target_dir.join(format!("{lib_name}.def")).display()
	));
	}

https://github.com/llvm/llvm-project/blob/llvmorg-19.1.6/lld/COFF/MinGW.cpp#L173-L192
https://github.com/bminor/binutils-gdb/blob/binutils-2_43_1/binutils/dlltool.c#L1813-L1872

Also bump implib while we are here, to ensure the import library works when targeting i686-pc-windows-gnu{,llvm}.

[sdroege]

CC @amyspark