n64sym: run objdump without a shell

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n64sym runs objdump through popen(), with $N64_INST and the ELF path pasted unquoted into the command line. A
space (or any other shell metacharacter) in either one breaks symbol generation:

$ N64_INST="/path/spaced dir/n64 sdk" n64sym game.elf game.sym
sh: /path/spaced: No such file or directory
sh: /path/spaced: No such file or directory
Error: objdump failed

$ n64sym "spaced dir/game.elf" game.sym
.../mips64-elf-objdump: 'spaced': No such file
.../mips64-elf-objdump: 'dir/game.elf': No such file
...
Error: objdump failed

addr2line was already run through subprocess_create() with an argument vector, so only the two objdump calls
(-t and -d) were affected.

This PR runs objdump the same way, through two small helpers: objdump_start() and objdump_finish().

• objdump still inherits the environment, as it did under popen().
• objdump's stderr is forwarded to n64sym's stderr after its output has been read, so errors such as
  file format not recognized still reach the user.
• The exit status check is now the same on every host, which removes the __MINGW32__ branch around pclose().
• With -v, the Running: lines are unchanged.

Testing was done on macOS with Apple clang. The tools build with -Werror. Each check ran on this branch and on
c79a52b42.

• The .sym output is byte-identical to the unfixed tool on three ROM ELFs. Each ELF was checked with the default
  options, --all, --no-inlines and --all --no-inlines.
• Each of those runs was repeated with a spaced N64_INST and a relative or absolute spaced ELF path. The output
  matches and stderr is empty.
• A relative N64_INST works, for example N64_INST=sdk.
• A path containing $(...) and quotes works.
• A missing objdump reports Error: cannot run: .../mips64-elf-objdump.
• A non-ELF input still prints objdump's error, followed by Error: objdump failed.

Windows (MinGW) was not built or tested. subprocess.h already handles argument quoting there for addr2line.

🤖 Generated with Claude Code

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Closing: this was opened prematurely on our side. Apologies for the noise.