Decompiler: avoid passing Sleigh IDs to BFD

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Summary:

• Treat an empty LoadImageBfd target as no explicit BFD target, passing NULL to bfd_openr().
• When BfdArchitecture receives an explicit Sleigh language id, avoid forwarding it to BFD as an object target name.
• Let BFD auto-detect the object format from the file header while resolveArchitecture() preserves the explicit Sleigh id.

Why:
resolveArchitecture() already treats colon-containing targets as Sleigh language ids. However, buildLoader() passed the same string through LoadImageBfd to bfd_openr(), where it is interpreted as a BFD target name. Values such as AARCH64:LE:64:v8A:default are valid Ghidra language ids, not BFD target names.

BFD distinguishes a NULL/default target from a literal empty target string. Passing NULL lets bfd_check_format() probe the supported object formats.

Testing:

• git diff --check