target/ppc: Simplify powerpc_excp_40x

target/ppc: Simplify powerpc_excp_40x

Differences from the generic powerpc_excp code:

- Not BookE, so some MSR bits are cleared at interrupt dispatch;
- No MSR_HV or MSR_LE;
- No power saving states;
- No Hypervisor Emulation Assistance;
- Not 64 bits;
- No System call vectored;
- No Interrupts Little Endian;
- No Alternate Interrupt Location.

Exceptions used:

POWERPC_EXCP_ALIGN
POWERPC_EXCP_CRITICAL
POWERPC_EXCP_DEBUG
POWERPC_EXCP_DSI
POWERPC_EXCP_DTLB
POWERPC_EXCP_EXTERNAL
POWERPC_EXCP_FIT
POWERPC_EXCP_ISI
POWERPC_EXCP_ITLB
POWERPC_EXCP_MCHECK
POWERPC_EXCP_PIT
POWERPC_EXCP_PROGRAM
POWERPC_EXCP_SYSCALL
POWERPC_EXCP_WDT

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20220118184448.852996-5-farosas@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>