[LLVMCPU] Default the export source location line to 0 (#25239)

LLVMCPUTarget fills an export's SourceLocation only when the debug level
is at least 1 and the export's location contains a file location.
Otherwise `line` stayed uninitialized and LibraryBuilder write random
bytes from the stack into the source location table of the executable
library.

The issue makes .vmfb files non-reproducible: compiling the same input
twice gives different bytes/checksums. It doesn't happen to all inputs,
only to some, e.g. TOSA bytecode from the TFLite converter, where every
op has `loc(unknown)`.

Default `line` to 0, like the other integer fields of structs in the
same module.

Tested by compiling four TFLite models multiple times with the llvm-cpu
backend and comparing binaries: before the change every new recompile
was different from the first one, after the fix all were identical.

Assisted-by: Claude Code

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Signed-off-by: Zmicier Prybysh <zprybysh@baylibre.com>