selftests/bpf: align build_id test mapping to 64K page size

selftests/bpf: align build_id test mapping to 64K page size

Some architectures require mappings to be aligned to the system page size.
The build_id selftest currently uses a smaller alignment, which can result
in madvise operations executing on a different page than intended.

Increase the mapping alignment to 64K so the buffer is page-aligned on
all supported architectures.

Signed-off-by: Gregory Bell <grbell@redhat.com>
Link: https://lore.kernel.org/r/93543253b32d1cb178ab6e31e4291e387ba1c372.1771338492.git.grbell@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>