[Hexagon] Fix compilation failures hexagon_remote

[aankit-ca]

Fix for:

1. Missing include directory for pthread.h
2. Incorrect function signature for qurt_hvx_lock

[aankit-ca]

@pranavb-ca @steven-johnson

[steven-johnson]

Pulling this into google3, it fails to build because known_symbols.cpp doesn't include qurt_hvx.h, which is needed for qurt_hvx_mode_t to be declared. If I add #include "qurt_hvx.h", it then fails to compile because the declarations in that header file aren't marked as extern "C".

How did you test this? Apparently we don't usually rebuild this file as part of our buildbots.

[steven-johnson]

...also, when I try to build locally, I fail for unrelated reasons:

# Hexagon SDK 4.3.0 is locally installed
$ export SDK_ROOT=$HOME/Qualcomm
$ export HEXAGON_SDK_ROOT=${SDK_ROOT}/Hexagon_SDK/4.3.0
$ export HEXAGON_TOOLS_ROOT=${HEXAGON_SDK_ROOT}/tools/HEXAGON_Tools/8.4.11
$ cd src/runtime/hexagon_remote
$ make all
/bin/sh: line 1: /usr/local/google/home/srj/Qualcomm/Hexagon_SDK/4.3.0/tools/android-ndk-r19c/toolchains/llvm/prebuilt/linux-x86_64/bin/clang++: No such file or directory