Add support for 64-bit PowerPC

[taylordotfish]

This PR adds bindings for 64-bit PowerPC (target_arch = "powerpc64"). I generated the bindings on a native ppc64le system, and in line with this commit, I added a repr(align(8)) to LV2_Atom.

[glaubitz]

This PR adds bindings for 64-bit PowerPC (target_arch = "powerpc64"). I generated the bindings on a native ppc64le system, and in line with this commit, I added a repr(align(8)) to LV2_Atom.

Can you explain how the bindings are generated? I used the command from sys/tool/README.md and it just fails with No such file or directory.

[taylordotfish]

@glaubitz You may have figured it out already, but I ran:

cd sys
LIBCLANG_PATH=/usr/lib/llvm-15/lib cargo systool --lv2 lv2/ --out src/linux/powerpc64.rs -- -v

This works around two issues: the first is that the older version of bindgen used by the tool doesn't work with LLVM 16 or newer, so I manually configured LIBCLANG_PATH for LLVM 15, and the second is that the tool panics if you don't provide any extra arguments to Clang, so I just added -v.

I then manually added repr(align(8)) to LV2_Atom in the generated output, to match what was done for other architectures.