x87: modelling of reading the status word wrong/confusing

Subject: https://github.com/rfalke/decompiler-subjects/blob/master/from_holdec/i386_x87_fpu/ia32_elf/subject.exe
Output:
```
int32_t TOP(void) {
// 0x804b710
__asm_wait();
int32_t v1; // 0x804b710
return (uint32_t)v1 / 2048 % 8;
}
```

This method returns 3 bits from the status word. The status word (v1) is not initialized. And it is not 32bit but 16bit wide.

See also https://github.com/uxmal/reko/issues/973 for a sister issue.