gdb: speed up lj-gctop command #2742

Job	Run time
LuaJIT (macOS/ARM64) Debug GC64:ON CC:clang	1m 6s
LuaJIT (macOS/ARM64) Release GC64:ON CC:clang	42s
LuaJIT (Linux/ARM64) Release GC64:ON CC:gcc	4m 40s
LuaJIT (Linux/ARM64) Debug GC64:ON CC:gcc	5m 1s
LuaJIT (Linux/x86_64) Debug GC64:ON CC:clang	3m 2s
LuaJIT (Linux/x86_64) Debug GC64:ON CC:gcc	3m 34s
LuaJIT (Linux/x86_64) Debug GC64:OFF CC:gcc	55s
LuaJIT (Linux/x86_64) Release GC64:ON CC:gcc	2m 30s
LuaJIT (Linux/x86_64) Release GC64:OFF CC:clang	42s
LuaJIT (Linux/x86_64) Release GC64:OFF CC:gcc	37s
LuaJIT (Linux/x86_64) Release GC64:ON CC:clang	2m 21s
LuaJIT (Linux/x86_64) Debug GC64:OFF CC:clang	53s
LuaJIT (macOS/x86_64) Debug GC64:ON CC:clang	1m 8s
LuaJIT (macOS/x86_64) Release GC64:ON CC:clang	50s
Tarantool (macOS/ARM64) Release GC64:ON / luajit-integration	2m 16s
Tarantool (Linux/ARM64) Release GC64:ON / luajit-integration	7m 45s
Tarantool (Linux/x86_64) Debug GC64:ON / luajit-integration	5m 28s
Tarantool (macOS/x86_64) Release GC64:ON / luajit-integration	4m 1s
Tarantool (macOS/x86_64) Debug GC64:ON / luajit-integration	2m 56s
Tarantool (Linux/x86_64) Debug GC64:OFF / luajit-integration	3m 52s
Tarantool (Linux/x86_64) Release GC64:OFF / luajit-integration	4m 22s
Tarantool (Linux/x86_64) Release GC64:ON / luajit-integration	5m 38s
Tarantool (macOS/ARM64) Debug GC64:ON / luajit-integration	2m 11s
Tarantool (Linux/ARM64) Debug GC64:ON / luajit-integration	10m 13s
	1h 16m 43s

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