imhex: fix LTO build with system LLVM demangle

[Rutpiv]

Testing the changes

• I tested the changes in this PR: briefly

Local build testing

• I built this PR locally for my native architecture (x86_64)

[Rutpiv]

The build fails when llvm18 is built with LTO enabled (-o lto), as pointed out by @JkktBkkt:
#58276 (comment)

This fails to build if llvm18 has been built with -o lto, solution is simple — rebuild llvm18 with -o ~lto.

I reproduced this locally by rebuilding llvm18 with LTO enabled. The failure is caused by an unresolved llvm::demangle symbol during linking. Even when ImHex is built with IMHEX_ENABLE_LTO, the demangle object is not pulled in when linking against system LLVM built with LTO.

This PR adds a small CMake adjustment to explicitly handle the demangle linkage when both IMHEX_ENABLE_LTO and USE_SYSTEM_LLVM are enabled. With this change, ImHex builds correctly against llvm18 built with LTO, without requiring rebuilding LLVM with ~lto.

Please let me know if there’s a more idiomatic way to handle this in CMake.