AIX FPU exception handling broken
## Summary
<!-- Summarize the bug encountered concisely -->
Startup code sets XE bit in $fpscr register,
leading to exception generation as soon as a FPU operation
sets "inexact result".
See:
`https://www.classe.cornell.edu/~cesrulib/documentation/totalview/html/Reference_Guides/wwhelp/wwhimpl/common/html/wwhelp.htm#href=UsingthePowerFPSCRRegister.html&single=true`
## System Information
<!-- The more information are provided the easier it is to replicate the bug -->
- **Operating system:**AIX <!-- Windows, Linux (if possible, also name the distro), FreeBSD, Android, ... -->
- **Processor architecture:** powerpc and powerpc64 <!-- x86, x86-64, ARM, AARCH64, AVR, RISC-V, PowerPC, ... -->
- **Compiler version:**3.3.1 <!-- 3.2, 3.2.2, 3.3, trunk, beta, ... (if possible, give also the git hash) -->
- **Device:** <!-- Computer, Tablet, Mobile, Amiga, Microcontroller, ... -->
## Steps to reproduce
<!-- How one can reproduce the issue - this is very important! -->
Compile and run source below.
## Example Project
<!-- If possible, please create an example project that exhibits the problematic
behavior, and link to it here in the bug report. -->
```
var
d1,d2 : double;
{$if defined(VER3_3) and not defined(AIX) }
fpscr: TNativeFPUControlWord;
{$else}
fpscr : DWord;
function getFPSCR : DWord; assembler; nostackframe;
asm
mffs f0
stfd f0, -12(r1)
lwz r3, -8(r1)
end;
{$endif}
begin
{$if defined(VER3_3) and not defined(AIX)}
fpscr:=GetNativeFPUControlWord;
{$else}
fpscr:=getFPSCR;
{$endif}
writeln('fpscr=$',hexstr(qword(fpscr),2*sizeof(qword)));
d1:=1.0;
d2:=155;
d2:=d1/d2;
writeln('d2=',d2);
end.
```
## What is the current bug behavior?
<!-- What actually happens -->
The test generates this output:
```
fpscr=$00000000000000F8
Runtime error 207 at $000000010000059C
```
## What is the expected (correct) behavior?
<!-- What you should see instead -->
On powerpc-linux or powerpc64-linux
fpscr is set to 090,
and in RTE 207 is generated.
AIX code should generate the same behavior.
## Relevant logs and/or screenshots
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## Possible fixes
<!-- If you can, link to the line of code that might be responsible for the problem -->
code in `rtl/ppcgen//ppcfpuex.inc` needs fixing.
issue