sort out librust_builtin placement

[aturon]

The test run-pass/sepcomp-lib-lto.rs can fail on android apparently due to the fact that a pthread shim dependency is present in libstd (as part of librust_builtin), but it stripped out early. It may be possible to work around this by moving the dependency into liballoc instead, but a more thorough examination is needed.

[aturon]

cc @alexcrichton

[alexcrichton]

The problem here on android was as follows:

• jemalloc requires the pthread_atfork symbol, which android does not provide. jemalloc is also statically linked to liballoc
• We provide a dummy definition of pthread_atfork in rust_builtin.c, which is assembled into librust_builtin.a and is statically linked to libstd.
• When building with LTO, we remove all rust-generated code from an rlib (it's all in the LTO object), but we still link the rlib.
• When linking, we pass libstd's rlib first, then liballoc's rlib on the command line.
• The linker detects that we're not using any symbols from the libstd's rlib by the time its reached on the command line (scans from left-to-right) because we actually aren't using any symbols from librust_builtin.a or any other objects in the archive.
• The linker then strips librust_builtin.a from the linkage step, which then later yields an unresolved reference.

The solution here is to break apart rust_builtin.c to be less of a conglomerate and instead link specific symbols to specific libraries wherever necessary. For example all of the time-related functionality should be a lib linked directly to libtime, not to libstd. In this example we need to link the pthread_atfork symbol directly to liballoc, not to libstd, because libstd is not the one that needs it.

Just some general cleanup, most things should still work as advertised!

[steveklabnik]

Triage: this test is still ignored on android, but I don't know what librust_builtin is nowadays.

[huonw]

I don't know what librust_builtin is nowadays.

Dead! So I guess this can be closed, but should we check if the affected test was actually fixed?

[alexcrichton]

Yeah I think this is all taken care of, so closing.