Added a "Building and Testing with an Emulator" section to
full_cross_build.rst using riscv64 and qemu-riscv64 as the example.
Outlined necessary CMake flags for cross-compiling with Clang, including
CMAKE_C_COMPILER_TARGET, CMAKE_CXX_COMPILER_TARGET, and
LLVM_ENABLE_LLD=ON. Switched from CMAKE_SYSROOT to LIBC_KERNEL_HEADERS
and added the gcc-riscv64-linux-gnu package dependency to ensure sysroot
issues on Debian-based systems are avoided while retaining access to
cross-compiler runtime objects.
Explained the self-hosted libc-hermetic-tests target as the required
target for executing tests during a standalone cross build, since the
standard check-libc tests are not hermetic.
Refactored existing CMake examples in full_cross_build.rst to use -S and
-B flags instead of cd and mkdir.
Removed prompt characters from code blocks and separated host
environment setup into explicit code blocks for easier copy-pasting.
Also removed the initial 'cd llvm-project' directory changes to reduce
boilerplate.
Added a cross-reference to the new emulator section in
build_and_test.rst.
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