mk: Build crates with relative source file paths
The path we pass to rustc will be visible in panic messages and
backtraces: they will be user visible!
Avoid junk in these paths by passing relative paths to rustc.
For most advanced users, `libcore` or `libstd` in the path will be
a clue to the location -- inside our code, not theirs.
Store both the relative path to the source as well as the absolute.
Use the relative path where it matters, compiling the main crates,
instead of changing all of the build process to cope with relative
paths.
Example output after this patch:
```
$ ./testunwrap
thread '<main>' panicked at 'called `Option::unwrap()` on a `None` value', ../src/libcore/option.rs:362
$ RUST_BACKTRACE=1 ./testunwrap
thread '<main>' panicked at 'called `Option::unwrap()` on a `None` value', ../src/libcore/option.rs:362
stack backtrace:
1: 0x7ff59c1e9956 - sys::backtrace::write::h67a542fd2b201576des
at ../src/libstd/sys/unix/backtrace.rs:158
2: 0x7ff59c1ed5b6 - panicking::on_panic::h3d21c41cdd5c12d41Xw
at ../src/libstd/panicking.rs:58
3: 0x7ff59c1e7b6e - rt::unwind::begin_unwind_inner::h9f3a5440cebb8baeLDw
at ../src/libstd/rt/unwind/mod.rs:273
4: 0x7ff59c1e7f84 - rt::unwind::begin_unwind_fmt::h4fe8a903e0c296b0RCw
at ../src/libstd/rt/unwind/mod.rs:212
5: 0x7ff59c1eced7 - rust_begin_unwind
6: 0x7ff59c22c11a - panicking::panic_fmt::h00b0cd49c98a9220i5B
at ../src/libcore/panicking.rs:64
7: 0x7ff59c22b9e0 - panicking::panic::hf549420c0ee03339P3B
at ../src/libcore/panicking.rs:45
8: 0x7ff59c1e621d - option::Option<T>::unwrap::h501963526474862829
9: 0x7ff59c1e61b1 - main::hb5c91ce92347d1e6eaa
10: 0x7ff59c1f1c18 - rust_try_inner
11: 0x7ff59c1f1c05 - rust_try
12: 0x7ff59c1ef374 - rt::lang_start::h7e51e19c6677cffe5Sw
at ../src/libstd/rt/unwind/mod.rs:147
at ../src/libstd/rt/unwind/mod.rs:130
at ../src/libstd/rt/mod.rs:128
13: 0x7ff59c1e628e - main
14: 0x7ff59b3f6b44 - __libc_start_main
15: 0x7ff59c1e6078 - <unknown>
16: 0x0 - <unknown>
```
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