libelf: Correctly setup alignment of SHF_COMPRESSED section data.

libelf: Correctly setup alignment of SHF_COMPRESSED section data.

We didn't set the alignment of SHF_COMPRESSED sections correctly.
Those sections start with an Elf(32|64)_Chdr. Make sure sh_addralign
is setup to be able to read such a struct directly. Likewise don't
trust the alignment set on any SHF_COMPRESSED section, but always
make the (raw) compressed data aligned correctly for the reading the
Elf(32|64)_Chdr directly.

Signed-off-by: Mark Wielaard <mark@klomp.org>