target/ppc: Set result to QNaN for DENBCD when VXCVI occurs

target/ppc: Set result to QNaN for DENBCD when VXCVI occurs

According to the ISA, for instruction DENBCD:
"If an invalid BCD digit or sign code is detected in the source
operand, an invalid-operation exception (VXCVI) occurs."

In the Invalid Operation Exception section, there is the situation:
"When Invalid Operation Exception is disabled (VE=0) and Invalid
Operation occurs (...) If the operation is an (...) or format the
target FPR is set to a Quiet NaN". This was not being done in
QEMU.

This patch sets the result to QNaN when the instruction DENBCD causes
an Invalid Operation Exception.

Signed-off-by: Víctor Colombo <victor.colombo@eldorado.org.br>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20220906125523.38765-5-victor.colombo@eldorado.org.br>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>