s390/bug: add entry size to the __bug_table section

Change the __EMIT_BUG inline assembly to emit mergeable __bug_table
entries with type @progbits and specify the size of each entry.
The entry size is encoded sh_entsize field of the section definition,
it allows to identify which struct bug_entry to use to decode the
entries. This will be needed for the objtool support.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>