Commit `083b4a3d66` introduced a store-and-load pair around the `BRASL`
call to mcount. That load instruction did not properly declare its
target register as defined, leading to a bad machine instruction.

This commit fixes this by explicitly labeling `%r14` on the load as
`def`.

When compiling with `-pg`, the `EntryExitInstrumenterPass` will insert
calls to the glibc function `mcount` at the begining of each
`MachineFunction`.

On SystemZ, these calls require special handling:

- The call to `mcount` needs to happen at the beginning of the prologue.
- Prior to the call to `mcount`, register `%r14`, the return address of
the callee function, must be stored 8 bytes above the stack pointer
`%r15`. After the call to `mcount` returns, that register needs to be
restored.

This commit adds some special handling to the EntryExitInstrumenterPass
that keeps the insertion of the mcount function into the module, but
skips over insertion of the actual call in order to perform this
insertion in the `emitPrologue` function. There, a simple sequence of
store/call/load is inserted, which implements the above.

The desired change in the `EntryExitInstrumenterPass` necessitated the
addition of a new attribute and attribute kind to each function, which
is used to trigger the postprocessing, aka call insertion, in
`emitPrologue`. Note that the new attribute must be of a different kind
than the `mcount` atribute, since otherwise it would replace that
attribute and later be deleted by the code that intended to delete
`mcount`. The new attribnute is called `insert-mcount`, while the
attribute kind is `systemz-backend`, to clearly mark it as a
SystemZ-specific backend concern.

This PR should address issue #121137 . The test inserted here is derived
from the example given in that issue.