Use BufWriter (instead of LineWriter) for FILEs other than stdout and stderr

See merge request redox-os/relibc!328

aarch64: TLS arch_read implementation

See merge request redox-os/relibc!330

Change ld.so linker script file name for x86_64 Linux to fix compilation issue

See merge request redox-os/relibc!329

BufWriter has more capacity (8k vs 1k) and doesn't flush the stream after '\n'.
That change helps to reduce the number of syscalls, especially when dealing with text files.

Since BufWriter has a different way of getting number of pending elements than LineWriter -
Pending trait was introduced to deal with that.

Fix dlsym of TLS variables

See merge request redox-os/relibc!327

Call DSO destructors during exit()

See merge request redox-os/relibc!326

Fix global symbols relocations

See merge request redox-os/relibc!325

inner_scanf prematurely exited before parsing collected string

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