pymupdfb: add build-pymupdfb.yml for riscv64 wheels

[luhenry]

• Package: pymupdfb
• Version: 1.24.10
• Source: https://github.com/pymupdf/PyMuPDF
• Docs: https://pymupdf.readthedocs.io/

Compiles MuPDF's shared libraries (libmupdf.so, libmupdfcpp.so) into the split wheel that PyMuPDF 1.24.10 depends on. Upstream publishes no riscv64 wheel.

Mirrors upstream's own scripts/gh_release.py, which builds this flavour with cibuildwheel and PYMUPDF_SETUP_FLAVOUR=bd; same in-tree pipcl/setup.py as build-pymupdf.yml (1.28.2), just a different flavour and an older tag.

Differs from upstream

• dnf install -y clang-devel + PYMUPDF_SETUP_LIBCLANG=clang - libclang PyPI wheel has no riscv64 build (same fix as build-pymupdf.yml).
• XCFLAGS/XCXXFLAGS=-D__LITTLE_ENDIAN__ - mupdf's tesseract endianness detection has no riscv64 case (same fix as build-pymupdf.yml).
• CIBW_REPAIR_WHEEL_COMMAND_LINUX='' - upstream itself disables repair for this flavour; auditwheel finds nothing to add and exits non-zero.
• musllinux dropped - same libclang-via-dnf workaround as build-pymupdf.yml, which is also manylinux-only.

Matrix: single cp312 selector - the wheel is py3-none-*, independent of Python version.

Testing: upstream never unit-tests this flavour standalone (only as a pip install fixture before testing PyMuPDF itself); the build instead asserts the wheel carries riscv64 ELF libmupdf.so/libmupdfcpp.so and its licence file.

License: OK

Built on cp312; wheel content and ELF machine type verified.