efi/cper: Fix cper_ia_proc_ctx alignment

According to the UEFI Common Platform Error Record appendix, the
IA32/X64 Processor Context Information Structure is a variable length
structure, but "is padded with zeros if the size is not a multiple
of 16 bytes".

Currently this isn't honoured, causing all but the first structure to
be garbage when printed. Thus align the size to be a multiple of 16.

Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>