Add support for dlopen(NULL, ...)

See merge request redox-os/relibc!315

Fix tls tests for dynamic linker

See merge request !317

* load TLS segment for executable - while we can skip PT_LOAD for executable,
  we still have to load TLS segment.
* set TCB address based on if elf is position independent

Add tests for ld_so

See merge request !314

Current value gives us 0x1234000 - 0x400000 ~ 14 MB for executable,
which is too low for certain programs.

Typically it's the other way around, but we can't have shared library named libc.so.6 in target/release directory.
cargo includes 'target/release' in LD_LIBRARY_PATH for build script, so even if clean build runs fine,
every subsquent run will make build script link with relibc.

Catch UB in asctime_r()

See merge request !312

Refactor gmtime_r(), fix localtime() test

See merge request !309

Add more gmtime() tests

See merge request redox-os/relibc!308

Avoid assuming c_char is i8 in asctime()

See merge request redox-os/relibc!307

Add needed include for asprintf() test

See merge request redox-os/relibc!306

link: Weaken __rust_probestack symbol

See merge request redox-os/relibc!310

Use renamed fmap call

See merge request redox-os/relibc!305

Emulate brk

See merge request redox-os/relibc!304

Gcc compile

See merge request redox-os/relibc!303

It seams that stdout of ld.so is not that much of an issue but actually
it unfortunately is. The major problem here is that sometimes programs
generate header files in stdout (./getmy_custom_headers > header.h) and
we need to keep that cleen. and this is very very popular in gcc.

This patch avoids collecting symbols, resolving relocs if they are
already done (usually for example libc.so during a dlopen for another
libfoo.so). This patch is purely for performance boost.

It is usually not optimal to load a library twice and for specifics,
it is **terrible** idea to load libc twice it was enough trouble
dealing with libc statically linked into ld.so. So What this patch does
it check for soname and if a library is already loaded it won't get
loaded again. Why soname ? because unfortunately some bins gets linked
againt libc.so while of their dependencies gets linked against
libc.so.6 while one is usually symbolic link for the other.