ASSERT - drmemtrace does not create the required pipe file if it is not present (Linux AARCH64).

The drmemtrace client uses a pipe to write its trace data. The pipe file is opened in drmemtrace_client_main() using dr_open_file(). dr_open_file() should create the file if it does not exist, according to the comments in dr_tools.h. However in the scenario below os_open() in drlibc_unix.c does not specify the O_CREAT flag.

**To Reproduce**
```
git clone https://github.com/DynamoRIO/dynamorio.git && cd dynamorio
git submodule init
git submodule update

cd ..
mkdir dynamorio_build
cd dynamorio_build
cmake -DBUILD_TESTS=ON ../dynamorio

gcc --version
# gcc (Ubuntu 11.4.0-2ubuntu1~20.04) 11.4.0

make -j

rm /tmp/drcachesimpipe

bin64/drrun -c clients/lib64/release/libdrmemtrace.so -- ls
# ASSERT FAILURE: ../dynamorio/clients/drcachesim/tracer/tracer.cpp:2340: fd != -1 ()

touch /tmp/drcachesimpipe

bin64/drrun -c clients/lib64/release/libdrmemtrace.so -- ls
# Normal output
```

**Versions**
- What version of DynamoRIO are you using?
latest in master (commit e9d8864931fb3b6f21983c07f0aa17f38e8706ec)

- What operating system version are you running on? ("Windows 10" is *not* sufficient: give the release number.)
Ubuntu 20.04.6 LTS

- Is your application 32-bit or 64-bit?
64-bit