Instead of a single source of symbols, now linker keeps a list of DSO (former Library) objects
with their own symbols map. That helps to process R_X86_64_COPY relocations correctly.
For example, if 'a.out' executable with dependencies ['libstdc++.so', 'libc.so'] is being loaded
and 'a.out' uses 'stdout' symbol from 'libc.so', its relocation process goes as follows:
- linker processes relocation entry 'stdout' of type R_X86_64_GLOB_DAT from 'libc.so',
- it goes through object list ['a.out', 'libstdc++.so', 'libc.so'] to find first object
  that exports 'stdout' symbol. The symbol is in 'a.out' with the value e.g. '0x404070',
- linker sets 'stdout' symbol GOT entry in 'libc.so' to '0x404070',
....
- linker processes relocation entry 'stdout' of type R_X86_64_COPY from 'a.out',
- it goes through object list excluding 'a.out': ['libstdc++.so', 'libc.so']. The symbol is found in 'libc.so',
- linker copies the 'stdout' symbol content from 'libc.so' to memory at address '0x404070' (in 'a.out' object).

Objects are relocated in reverse order they were loaded. So in the example above, linker starts with relocating
'libc.so' and ends with 'a.out'. It is necessary e.g. when linking with 'libstdc++.so' - there are many
relocations which symbols are found in 'libstdc++.so', so they need to be resolved before their contents are
copied to 'a.out'. That also matches GNU ld.so behavior.

Add submodules target to main makefile

See merge request redox-os/relibc!323

Adds a submodules convenience target to main makefile. Submodules must
be initialized before other targets can run successfully.

Signed-off-by: hasheddan <georgedanielmangum@gmail.com>

Fix formatiing issues

See merge request !316

Add support for dlopen(NULL, ...)

See merge request !315

Fix tls tests for dynamic linker

See merge request redox-os/relibc!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 redox-os/relibc!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 redox-os/relibc!312

Refactor gmtime_r(), fix localtime() test

See merge request redox-os/relibc!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