Implement .eh_frame writing

[bjorn3]

thread 'main' panicked at 'explicit panic', example/std_example.rs:8:5
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /home/bjorn/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /home/bjorn/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at sysroot_src/src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at sysroot_src/src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at sysroot_src/src/libcore/fmt/mod.rs:1069
   5: std::io::Write::write_fmt
             at sysroot_src/src/libstd/io/mod.rs:1504
   6: std::sys_common::backtrace::_print
             at sysroot_src/src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at sysroot_src/src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at sysroot_src/src/libstd/panicking.rs:198
   9: std::panicking::default_hook
             at sysroot_src/src/libstd/panicking.rs:218
  10: std::panicking::rust_panic_with_hook
             at sysroot_src/src/libstd/panicking.rs:511
  11: std::panicking::begin_panic
             at sysroot_src/src/libstd/panicking.rs:438
  12: std_example::main
             at example/std_example.rs:8
  13: std::rt::lang_start::{{closure}}
             at ./build_sysroot/sysroot_src/src/libstd/rt.rs:67
  14: std::rt::lang_start_internal::{{closure}}::{{closure}}
             at sysroot_src/src/libstd/rt.rs:52

This currently requires debuginfo to be enabled. It also doesn't work for jitted code.