Removed the stray build artifacts committed with the RISC-V64 port fix

[fdesbiens]

A local CMake build tree was staged by mistake alongside the RISC-V64 spec compliance work in #698, so nine generated files landed on dev:

File	Size
build-rv64/build.ninja	444 KB
build-rv64/ports/risc-v64/gnu/example_build/qemu_virt/kernel.elf	239 KB
build-rv64/.ninja_deps	88 KB
build-rv64/.ninja_log	43 KB
build-rv64/custom_inc/tx_user.h	11 KB
build-rv64/ports/risc-v64/gnu/example_build/qemu_virt/qemu-riscv64.log	8 KB
build-rv64/ports/risc-v64/gnu/example_build/qemu_virt/test_cmds.gdb	6 KB
build-rv64/CPackSourceConfig.cmake	4 KB
build-rv64/CPackConfig.cmake	4 KB

None of it belongs in the repository, and none of it is part of the RISC-V64 fix itself — that change is correct and stays as merged.

Why the ignore rules did not catch it

The root .gitignore listed build directories by name rather than by pattern:

build/
build_qemu/
build_m7/
build_r52/
build_r52_vfp/
build_r52_uart/
build_r52_mpu/
build_r52_all/

Every one of those is a name someone happened to use. A tree configured under any other name was staged normally. Those eight entries are replaced with a single build*/, which covers all of them and anything future. I confirmed no tracked file matches the new pattern:

git ls-files | git check-ignore --stdin   # no output

example_build/ directories are unaffected, since the pattern anchors on names beginning with build.

On history

The blobs stay reachable in dev's history. Only the working tree is corrected — rewriting a shared branch that contributors have already pulled is the greater harm for 830 KB of dead objects.