Print panic backtraces on Linux

[Kha]

Error: index out of bounds
backtrace:
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(lean_panic_fn+0x76)[0x7f941ad473d6]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(l_Array_back___rarg___boxed+0x60)[0x7f94189de830]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(+0x29bc986)[0x7f941ad40986]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(+0x29baab7)[0x7f941ad3eab7]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(+0x29b812b)[0x7f941ad3c12b]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(+0x29baab7)[0x7f941ad3eab7]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(+0x29bcd37)[0x7f941ad40d37]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(+0x29baab7)[0x7f941ad3eab7]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(+0x29be72e)[0x7f941ad4272e]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(+0x29be322)[0x7f941ad42322]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(+0x29bdebf)[0x7f941ad41ebf]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(lean_apply_1+0x4a4)[0x7f941ad504a4]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(l_Lean_instEval__1___rarg___boxed+0x2c)[0x7f9418b654dc]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(lean_apply_3+0x628)[0x7f941ad523b8]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(l_Lean_runEval___rarg___lambda__1+0x15)[0x7f9418b66cb5]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(lean_apply_1+0x16a)[0x7f941ad5016a]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(l_IO_withStderr___at_Lean_runEval___spec__4+0x6e)[0x7f9418b660fe]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(lean_apply_1+0x16a)[0x7f941ad5016a]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(l_IO_withStdout___at_Lean_runEval___spec__2+0x6e)[0x7f9418b659fe]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(lean_apply_1+0x16a)[0x7f941ad5016a]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(l_IO_withStdin___at_Lean_runEval___spec__3+0x6e)[0x7f9418b65d7e]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(l_IO_FS_withIsolatedStreams___at_Lean_runEval___spec__1+0x192)[0x7f9418b665a2]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(+0x29bc986)[0x7f941ad40986]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(+0x29baab7)[0x7f941ad3eab7]
/nix/store/zhf1q7di0ggl1qawsikc8v98x9yy53c4-leanshared/libleanshared.so(+0x29be72e)[0x7f941ad4272e]
...

I'm sure libunwind would do a better job, and across more platforms, but this was a low-hanging fruit.