fix: Fix the libbacktrace cfg for nicer backtraces

[tibordp]

The cfg was renamed from use_libbacktrace to just libbacktrace.

Before

panic at program.alu:7:9: oops, I'm a bad comparer
./program.out(_AL19print_panic_message172+0x38c)[0x55bc50ebecc0]
./program.out(_AL10panic_impl250+0x150)[0x55bc50ec1f9d]
./program.out(_AL14chaos_comparer204+0xec)[0x55bc50ec1178]
./program.out(_AL12partition_by314+0x21e9)[0x55bc50ec4990]
./program.out(_AL12introsort_by111+0xd1)[0x55bc50ebe112]
./program.out(_AL7sort_by58+0x47)[0x55bc50ebd025]
./program.out(_AL4main0+0xa6)[0x55bc50ebc3af]
./program.out(main+0x27)[0x55bc50ec1fcc]
/lib/x86_64-linux-gnu/libc.so.6(+0x2a1ca)[0x7fb1e782a1ca]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x8b)[0x7fb1e782a28b]
./program.out(_start+0x25)[0x55bc50ebc245]

After

panic at program.alu:7:9: oops, I'm a bad comparer
stack backtrace:
--> chaos_comparer at /tmp/tmp-1-7l5EGZuK7fqp/program.alu:7
--> partition_by at /usr/include/alumina/std/cmp.alu:724
--> introsort_by at /usr/include/alumina/std/cmp.alu:798
--> sort_by at /usr/include/alumina/std/cmp.alu:461
--> main at /tmp/tmp-1-7l5EGZuK7fqp/program.alu:20
--> main at /usr/include/alumina/std/runtime/mod.alu:416
--> <unknown>
--> <unknown>
--> <unknown>
--> <unknown>