common parts between C/gcc-c-torture/CMakeLists.txt and the new
C/gcc-dg/CMakeLists.txt are put in DgOptions.txt

The `pr89313.c` and `pr87600-1.c` tests were passing fine on LLVM
trunk (specifically I was on `c1eef483b2c1`), but failed on 17.0.5.

I bisected the failure down to the following LLVM commit:

d79ccee8dc4a [X86][MC] Support encoding/decoding for APX variant ADD/SUB/ADC/SBB/OR/XOR/NEG/NOT instructions (#76319)

So it turns out that the three-operand form of `add`, on x86_64, is
actually a very recent addition.

We should therefore probably avoid using it incase someone wants to
run against an older but still recent LLVM version.

RISC-V does not support the __bf16 datatype, therefore disable test.

GCC has the notion of "default test behaviour", which is stored in the
`dg-do-what-default` global variable. It specified what should happen
when there is no `dg-do` comment found in the test suite.

Our current behaviour is that if there's no `dg-do`, then we just
compile the test. But for the `gcc.c-torture/execute` directory, the
default behaviour is to run the test, because the `execute.exp` script
manaully sets it so:

```
set saved-dg-do-what-default ${dg-do-what-default}
set dg-do-what-default "run"
gcc-dg-runtest [lsort [glob -nocomplain $srcdir/$subdir/*.\[cS\]]] "" "-w"
set dg-do-what-default ${saved-dg-do-what-default}
```

We should probably model this behaviour in our CMake scripts, but for
now undo the refactoring in the execute/CMakeLists.txt script, and for
tests in that particular directory we always try to run the file.

In the GCC test suite many tests don't have an explicit `dg-do`
directive telling the test driver what to do. In these cases, the
default action is taken from a TCL variable called
`dg-do-what-default`, which can be set freely (see for example
gcc/testsuite/gcc.c-torture/execute/execute.exp)

Emulate this behaviour in our CMakeScripts also.