target/ppc: Simplify powerpc_excp_74xx

target/ppc: Simplify powerpc_excp_74xx

Differences from the generic powerpc_excp code:

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

Exceptions used:

POWERPC_EXCP_ALIGN
POWERPC_EXCP_DECR
POWERPC_EXCP_DSI
POWERPC_EXCP_EXTERNAL
POWERPC_EXCP_FPU
POWERPC_EXCP_IABR
POWERPC_EXCP_ISI
POWERPC_EXCP_MCHECK
POWERPC_EXCP_PERFM
POWERPC_EXCP_PROGRAM
POWERPC_EXCP_RESET
POWERPC_EXCP_SMI
POWERPC_EXCP_SYSCALL
POWERPC_EXCP_THERM
POWERPC_EXCP_TRACE
POWERPC_EXCP_VPU
POWERPC_EXCP_VPUA

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Message-Id: <20220127201116.1154733-3-farosas@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>