add aarch64_unknown_nto_qnx700 target - QNX 7.0 support for aarch64le

[nyurik]

This backports the QNX 7.1 aarch64 implementation to 7.0.

• required -lregex disabled, see Disable libregex for QNX 7.0 libc#3775 (released in libc 0.2.156)
• uses libgcc.a instead of libgcc_s.so (7.0 used ancient GCC 5.4 which didn't have gcc_s)
• a fix in backtrace crate to support stack traces Add QNX NTO platform support backtrace-rs#648

This PR bumps libc dependency to 0.2.158

CC: to the folks who did the initial implementation: @flba-eb, @gh-tr, @jonathanpallant, @japaric

Compile target

Note that until qnx700 gets properly released, use BOOTSTRAP_SKIP_TARGET_SANITY=1

# Configure qcc build environment
source _path_/_to_/qnx7.0/qnxsdp-env.sh

# Tell rust to use qcc when building QNX 7.0 targets
export build_env='
    BOOTSTRAP_SKIP_TARGET_SANITY=1
    CC_aarch64-unknown-nto-qnx700=qcc
    CFLAGS_aarch64-unknown-nto-qnx700=-Vgcc_ntoaarch64le_cxx
    CXX_aarch64-unknown-nto-qnx700=qcc
    AR_aarch64_unknown_nto_qnx700=ntoaarch64-ar'

# Build rust compiler, libs, and the remote test server
env $build_env ./x.py build \
  --target x86_64-unknown-linux-gnu,aarch64-unknown-nto-qnx700 \
  rustc library/core library/alloc library/std src/tools/remote-test-server

rustup toolchain link stage1 build/host/stage1

Compile "hello world"

source _path_/_to_/qnx7.0/qnxsdp-env.sh

cargo new hello_world
cd hello_world
cargo +stage1 build --release --target aarch64-unknown-nto-qnx700

Configure a remote for testing

Do this from a new shell - we will need to run more commands in the previous one. I ran into these two issues, and found some workarounds.

• Temporary dir might not work properly
• Default remote-test-server has issues binding to an address

# ./remote-test-server
starting test server
thread 'main' panicked at src/tools/remote-test-server/src/main.rs:175:29:
called `Result::unwrap()` on an `Err` value: Os { code: 249, kind: AddrNotAvailable, message: "Can't assign requested address" }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Specifying --bind param actually fixes that, and so does setting TMPDIR properly.

# Copy remote-test-server to remote device. You may need to use sftp instead.
# ATTENTION: Note that the path is different from the one in the remote testing documentation for some reason
scp ./build/x86_64-unknown-linux-gnu/stage1-tools-bin/remote-test-server  qnxdevice:/path/

# Connect to the remote device to start the remote tester
# The port forwarding (-L) is only needed if the device is not accessible directly via TCP/IP.
# Port forwarding allows the tester to connect to the local port instead
ssh -L 12345:127.0.0.1:12345 qnxdevice

# on the device, run
rm -rf tmp && mkdir -p tmp && TMPDIR=$PWD/tmp ./remote-test-server --bind 0.0.0.0:12345

Run test suit

Assume all previous environment variables are still set, or re-init them

export TEST_DEVICE_ADDR="localhost:12345"

# tidy needs to be skipped due to using un-published libc dependency
export exclude_tests='
    --exclude src/bootstrap
    --exclude src/tools/error_index_generator
    --exclude src/tools/linkchecker
    --exclude src/tools/tidy
    --exclude tests/ui-fulldeps
    --exclude rustc
    --exclude rustdoc
    --exclude tests/run-make-fulldeps'

env $build_env ./x.py test  $exclude_tests --stage 1 --target aarch64-unknown-nto-qnx700

try-job: dist-x86_64-msvc