Add Intel CET to getTargetInfo (#15433)

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.

changelog/dmd.intel-cet-ibt-protection.dd

@@ -0,0 +1,22 @@
+Added support for Intel CET (Control-flow Enforcement Technology) IBT (Indirect Branch Tracking) protection
+
+CET is a technology that is useful for preventing an attacker from redirecting a program's control flow,
+specifically IBT prevents an attacker from causing an indirect branch to go to an unintended place.
+
+Intel IBT expects the compiler to emit special instructions (`endbr32` and `endbr64`) which in older processors
+that do not support IBT are equivalent to `nop` instructions, consequently a program compiled with active IBT
+will be compatible on any x86 processor and the protection will be opportunistically active on supported processors.
+
+To enable Intel IBT protection in DMD you need to pass the `-fIBT` flag to the compiler, consequently the compiler
+will manage the emission of instructions for IBT by itself.
+Be careful when using inline assembly, the compiler will not automatically handle IBT inside an inline assembly.
+
+To find out within a D program whether IBT has been activated or not use the traits getTargetInfo as follows:
+
+---
+// IBT active
+static assert(__traits(getTargetInfo, "CET") == 1);  // CET == 1 if IBT is active
+
+// IBT not active
+static assert(__traits(getTargetInfo, "CET") == 0);  // CET == 0 if IBT is not active
+---

compiler/src/dmd/frontend.h

@@ -8284,6 +8284,7 @@ private:
         cppStd = 1,
         floatAbi = 2,
         objectFormat = 3,
+        CET = 4,
     };
 
 public:

compiler/src/dmd/target.d

@@ -1206,6 +1206,7 @@ extern (C++) struct Target
         cppStd,
         floatAbi,
         objectFormat,
+        CET
     }
 
     /**
@@ -1248,6 +1249,8 @@ extern (C++) struct Target
                 return stringExp("");
             case cppStd.stringof:
                 return new IntegerExp(params.cplusplus);
+            case CET.stringof:
+                return new IntegerExp(driverParams.ibt);
 
             default:
                 return null;

compiler/test/compilable/cet_disabled.d

@@ -0,0 +1,3 @@
+// Test for Intel CET protection disabled
+
+static assert(__traits(getTargetInfo, "CET") == 0);

compiler/test/compilable/cet_ibt.d

@@ -0,0 +1,5 @@
+// REQUIRED_ARGS: -fIBT
+
+// Test for Intel CET IBT (branch) protection
+
+static assert(__traits(getTargetInfo, "CET") == 1);