Properly free module info when using linker-generated section brackets

[joakim-noah]

The recently-merged support for linker-generated section brackets (#1379) causes a segfault on Android, when rt_term tries to free module info that is no longer malloced. I'm guessing it broke for Solaris too, though I haven't tested it. This PR seems to fix it- all druntime and phobos tests on Android/x86 and linux/x86 pass locally- let me know if any further tweaks are necessary.

[joakim-noah]

Looks like setting _modules to null in rt.minfo was causing a segfault in the debug build on linux, so I got rid of it. I had only tested locally with a release build, which is why I didn't see the issue, but I can reproduce with a debug build. The cause is the invariant in rt.sections_elf_shared, which is called after the modules are removed, likely when the destructor is run. I don't know why it only triggered on one module from the debug build, std.parallelism, but getting rid of the null assignment should fix the issue.

[joakim-noah]

As @MartinNowak asked for in a previously-merged PR thread, moved some non-Posix bionic declarations into their own directory.

[dnadlinger]

@redstar: IIRC you are testing Solaris?

[MartinNowak]

Auto-merge toggled on

[redstar]

@klickverbot I'll check this on Solaris.