lib/buildid: take into account e_phoff when fetching program headers

Current code assumption is that program (segment) headers are following
ELF header immediately. This is a common case, but is not guaranteed. So
take into account e_phoff field of the ELF header when accessing program
headers.

Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>
Reported-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240829174232.3133883-4-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>