Add Intel CET to getTargetInfo

[ErnyTech]

This follows PR #15415 which added Intel CET IBT support and LDC PR #4437 to add support for the new CET target in order to maintain a common interface between DMD and LDC.

Later it would be useful to do the same for GDC as well.

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Testing this PR locally

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[ghost]

I suggest to use "intelCET" instead of just "CET" as the later is not very well known.

[dkorpel]

Why does D code need to know about this setting?

[ghost]

Why does D code need to know about this setting?

inline asm maybe ?

[ErnyTech]

Why does D code need to know about this setting?

Mainly for inline ASM.

I suggest to use "intelCET" instead of just "CET" as the later is not very well known.

C compilers use __CET__, I think CET is clear enough for those who have already used Intel CET

[RazvanN7]

Please add a changelog entry describing this change so that users get to know about this.

[ErnyTech]

Please add a changelog entry describing this change so that users get to know about this.

Added

satisfactory explanation

[ErnyTech]

@dkorpel Is there anything blocking this PR? I think it would be good to include it in 2.106.0-beta especially since it also provides the changelog for -fIBT
Walter's request seems satisfied

[ibuclaw]

This test is non-portable, and impossible to downstream into gdc (and I assume ldc must be the same as well). Maybe there should be an x86 directory added for such tests - this would include all DMD asm tests too.