[llvm-objcopy] llvm-objcopy --change-section-lma shifts unaddressed segments (e.g. PT_RISCV_ATTRIBUTES), causing spurious underflow

`--change-section-lma` shifts every segment with `MemSize > 0`, without checking whether the segment was ever assigned a real address. A segment that legitimately has `VAddr == PAddr == 0` (never placed in memory by the linker) but a nonzero size — e.g. a `PT_RISCV_ATTRIBUTES` segment — gets shifted anyway, and a large-enough negative shift underflows and aborts the entire operation, even though other real segments in the same file would have shifted correctly.

### Repro
```
$ cat repro.yaml
!ELF
FileHeader:
Class: ELFCLASS64
Data: ELFDATA2LSB
Type: ET_EXEC
Sections:
- Name: .text
Type: SHT_PROGBITS
Size: 0x10
- Name: .riscv.attributes
Type: SHT_PROGBITS
Size: 0x38
ProgramHeaders:
- Type: PT_LOAD
VAddr: 0x1000
FirstSec: .text
LastSec: .text
- Type: 0x70000003 # PT_RISCV_ATTRIBUTES
Flags: [ PF_R ]
VAddr: 0x0
PAddr: 0x0
Align: 0x1
FirstSec: .riscv.attributes
LastSec: .riscv.attributes

$ yaml2obj repro.yaml -o repro.elf
$ llvm-objcopy --change-section-lma *-0x30 repro.elf shifted.elf
error: address 0x0 cannot be decreased by 0x30. The result would underflow
```

I discovered the problem while trying to build a real-world RISC-V ELF (a Zephyr RTOS build for a RISC-V VPR coprocessor core) containing a genuine PT_RISCV_ATTRIBUTES segment using an LLVM toolchain built from the current main (24.0.0). The problem also reproduces with LLVM 19.1.7 that is bundled with Zephyr-SDK-1.0.1. and on current main.

If I configure my build to use the GNU objcopy (also bundled with the same Zephyr SDK) the problem doesn't arise and I get a working executable.

I used Claude to analyze the problem and have created a PR (#218414) with a proposed fix + a new test. The fix is very small and solves the Zephyr problem and passes the llvm-objcopy tests, so it seems that nothing obvious is broken. As I am not versed in the llvm-objcopy nor ELFs there can obviously be something that I don't understand. There seems to have been a previous issue (#124680) that made changes to `--change-section-lma`

### Expected
A segment with no real address `(VAddr == 0 && PAddr == 0)` shouldn't participate in the shift, regardless of its size.

Start with llvm-objcopy's --change-section-lma handling and reproduce the failure using repro.yaml, yaml2obj, and the command shown. Confirm that the unaddressed PT_RISCV_ATTRIBUTES segment does not participate in the shift, while real segments still shift and the operation completes without an underflow error.

Written by the indexing model from the issue text.