[SystemZ] [z/OS] Emit offset to PPA2 in separate MCSection (#84043)

The ppa2list section isn't really part of the ppa2 section. The ppa2list
section contains the offset to the ppa2, and must be created with a
special section name (specifically, C_@@QPPA2). The binder searches for
a section with this name, then uses this value to locate the ppa2.

In GOFF terms, these are entirely separate sections; the PPA2 section
isn't even really a section but rather belongs to the code section. On
the other hand, the ppa2list section is a section in its own right and
resides in a separate TXT record.