[Bazel] Generate LLVM_HAS_XYZ_TARGET macros in llvm config (#94476)

Otherwise code that depends on those targets being enabled might not get
compiled correctly even if the targets are explicitly included in the
configuration (in my case NVVM target for MLIR).

Author: apaszke

utils/bazel/llvm-project-overlay/llvm/BUILD.bazel

@@ -164,14 +164,18 @@ td_library(
     includes = ["include"],
 )
 
+llvm_config_target_defines = [
+  "LLVM_HAS_{}_TARGET=1".format(t) for t in llvm_targets
+]
+
 cc_library(
     name = "config",
     hdrs = [
         "include/llvm/Config/abi-breaking.h",
         "include/llvm/Config/llvm-config.h",
     ],
     copts = llvm_copts,
-    defines = llvm_config_defines,
+    defines = llvm_config_defines + llvm_config_target_defines,
     includes = ["include"],
     textual_hdrs = [
         "include/llvm/Config/AsmParsers.def",