[libc] Rename pthread_impl.h to threads_impl.h

Baby step towards separating pthreads out of libc

Change-Id: Ibb1ba6656c796218909f4a3451db9bb66b99fb80

[musl][atomics] Use C11 atomics in public types

Change-Id: Ie684384c127d881fbeccb1290adce622d368d136

Revert "[musl][atomics] Use C11 atomics in public types"

This reverts commits f2373edae22bc921a41286318243b3985102b510 and
c97bbfb0755f81841769a12a12ba5fbe86144a6b

This breaks libc++ builds:

FAILED: src/CMakeFiles/unwind_objects.dir/libunwind.cpp.o
/b/swarm_slave/w/irzvTVgH/kitchen-workdir/buildtools/toolchain/clang+llvm-x86_64-linux/bin/clang++ -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/b/swarm_slave/w/irzvTVgH/kitchen-workdir/third_party/llvm/runtimes/libunwind/include -I/b/swarm_slave/w/irzvTVgH/kitchen-workdir/third_party/llvm/runtimes/libcxx/include -stdlib=libc++ -fvisibility-inlines-hidden -Werror=date-time -std=c++11 -fcolor-diagnostics -target x86_64-fuchsia --sysroot=/b/swarm_slave/w/irzvTVgH/kitchen-workdir/out/sysroot/x86_64-fuchsia -std=c++11 -g -Werror=return-type -W -Wall -Wchar-subscripts -Wconversion -Wmismatched-tags -Wmissing-braces -Wnewline-eof -Wno-unused-function -Wshadow -Wshorten-64-to-32 -Wsign-compare -Wsign-conversion -Wstrict-aliasing=2 -Wstrict-overflow=4 -Wunused-parameter -Wunused-variable -Wwrite-strings -Wundef -Wno-error -pedantic -D_DEBUG -D_LIBUNWIND_IS_NATIVE_ONLY -fPIC -fno-exceptions -funwind-tables -I/b/swarm_slave/w/irzvTVgH/kitchen-workdir/third_party/llvm/runtimes/libcxx/include -stdlib=libc++ -fvisibility-inlines-hidden -Werror=date-time -std=c++11 -fcolor-diagnostics -target x86_64-fuchsia --sysroot=/b/swarm_slave/w/irzvTVgH/kitchen-workdir/out/sysroot/x86_64-fuchsia -std=c++11 -fstrict-aliasing -fno-rtti -MD -MT src/CMakeFiles/unwind_objects.dir/libunwind.cpp.o -MF src/CMakeFiles/unwind_objects.dir/libunwind.cpp.o.d -o src/CMakeFiles/unwind_objects.dir/libunwind.cpp.o -c /b/swarm_slave/w/irzvTVgH/kitchen-workdir/third_party/llvm/runtimes/libunwind/src/libunwind.cpp
In file included from /b/swarm_slave/w/irzvTVgH/kitchen-workdir/third_party/llvm/runtimes/libunwind/src/libunwind.cpp:19:
In file included from /b/swarm_slave/w/irzvTVgH/kitchen-workdir/third_party/llvm/runtimes/libcxx/include/algorithm:640:
In file included from /b/swarm_slave/w/irzvTVgH/kitchen-workdir/third_party/llvm/runtimes/libcxx/include/memory:643:
/b/swarm_slave/w/irzvTVgH/kitchen-workdir/third_party/llvm/runtimes/libcxx/include/atomic:1713:17: error: address argument to atomic operation must be a pointer to _Atomic type ('volatile bool *' invalid)
{return __c11_atomic_exchange(&__a_, true, __m);}
^ ~~~~~
/b/swarm_slave/w/irzvTVgH/kitchen-workdir/third_party/llvm/runtimes/libcxx/include/atomic:1716:17: error: address argument to atomic operation must be a pointer to _Atomic type ('bool *' invalid)
{return __c11_atomic_exchange(&__a_, true, __m);}
^ ~~~~~
/b/swarm_slave/w/irzvTVgH/kitchen-workdir/third_party/llvm/runtimes/libcxx/include/atomic:1719:10: error: address argument to atomic operation must be a pointer to _Atomic type ('volatile bool *' invalid)
{__c11_atomic_store(&__a_, false, __m);}
^ ~~~~~
/b/swarm_slave/w/irzvTVgH/kitchen-workdir/third_party/llvm/runtimes/libcxx/include/atomic:1722:10: error: address argument to atomic operation must be a pointer to _Atomic type ('bool *' invalid)
{__c11_atomic_store(&__a_, false, __m);}

Change-Id: I2fba5de2b1abc16f6e2ab3ae11f4b6ee1e70f135

[musl][atomics] Use C11 atomics in public types

For now, use a shim around compare and swap. Followup patches will
rewrite them directly into the C11 style, but for now minimize noise.

Change-Id: Idf3626fea6318b8508d6e65f2634a71e93cf866c

[musl] Pull pthreads source files into a separate directory

Change-Id: Ic34f0c5c18cf50e42291342e978169ebf1130ae1