Merge pull request #20392 from fpetrogalli:aarch64-semihosting

Merge pull request #20392 from fpetrogalli:aarch64-semihosting

AArch64 semihosting

* [ts] Disable filesystem support in the TS module.

Because of this change, all the tests loading data will file, but tat
least the core module can be tested with the following line:

    opencv_test_core --gtest_filter=-"*Core_InputOutput*:*Core_globbing.accuracy*"

* [aarch64] Build OpenCV for AArch64 semihosting.

This patch provide a toolchain file that allows to build the library
for semihosting applications [1]. Minimal changes have been applied to
the code to be able to compile with a baremetal toolchain.

[1] https://developer.arm.com/documentation/100863/latest

The option `CV_SEMIHOSTING` is used to guard the bits in the code that
are specific to the target.

To build the code:

    cmake ../opencv/ \
        -DCMAKE_TOOLCHAIN_FILE=../opencv/platforms/semihosting/aarch64-semihosting.toolchain.cmake \
        -DSEMIHOSTING_TOOLCHAIN_PATH=/path/to/baremetal-toolchain/bin/ \
        -DBUILD_EXAMPLES=ON -GNinja

A barematel toolchain for targeting aarch64 semihosting can be found
at [2], under `aarch64-none-elf`.

[2] https://developer.arm.com/tools-and-software/open-source-software/developer-tools/gnu-toolchain/gnu-a/downloads

The folder `samples/semihosting` provides two example semihosting
applications.

The two binaries can be executed on the host platform with:

    qemu-aarch64 ./bin/example_semihosting_histogram
    qemu-aarch64 ./bin/example_semihosting_norm

Similarly, the test and perf executables of the modules can be run
with:

    qemu-aarch64 ./bin/opecv_[test|perf]_<module>

Notice that filesystem support is disabled by the toolchain file,
hence some of the test that depend on filesystem support will fail.

* [semihosting] Remove blank like at the end of file. [NFC]

The spurious blankline was reported by
https://pullrequest.opencv.org/buildbot/builders/precommit_docs/builds/31158.

* [semihosting] Make the raw pixel file generation OS independent.

Use the facilities provided by Cmake to generate the header file
instead of a shell script, so that the build doesn't fail on systems
that do not have a unix shell.

* [semihosting] Rename variable for semihosting compilation.

* [semihosting] Move the cmake configuration to a variable file.

* [semihosting] Make the guard macro private for the core module.

* [semihosting] Remove space. [NFC]

* [semihosting] Improve comment with information about semihosting. [NFC]

* [semihosting] Update license statement on top of sourvce file. [NFC]

* [semihosting] Replace BM_SUFFIX with SEMIHOSTING_SUFFIX. [NFC]

* [semihosting] Remove double space. [NFC]

* [semihosting] Add some text output to the sample applications.

* [semihosting] Remove duplicate entry in cmake configuration. [NFCI]

* [semihosting] Replace `long` with `int` in sample apps. [NFCI]

* [semihosting] Use `configure_file` to create the random pixels. [NFCI]

* [semihosting][bugfix] Fix name of cmakedefine variable.

* [semihosting][samples] Use CV_8UC1 for grayscale images. [NFCI]

* [semihosting] Add readme file.

* [semihosting] Remove blank like at the end of README. [NFC]

This fixes the failure at
https://pullrequest.opencv.org/buildbot/builders/precommit_docs/builds/31272.