Migrate tests and benchmarks from `--iree-llvmcpu-enable-microkernels` to `--iree-llvmcpu-enable-ukernels` (#15584)

Following https://github.com/openxla/iree/pull/15571, we migrate
existing tests and benchmarks from the old deprecated flag to the new
one.

This will allow us to drop `--iree-llvmcpu-enable-microkernels` in a
future PR.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0110c17..bc4a4d8 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -183,7 +183,7 @@
 # Experimental project flags
 #-------------------------------------------------------------------------------
 
-option(IREE_BUILD_EXPERIMENTAL_CPU_UKERNEL_PLUGINS "Build experimental plugins making builtin ukernels available to llvm-cpu modules compiled with --iree-llvmcpu-enable-microkernels" OFF)
+option(IREE_BUILD_EXPERIMENTAL_CPU_UKERNEL_PLUGINS "Build experimental plugins making builtin ukernels available to llvm-cpu modules compiled with --iree-llvmcpu-enable-ukernels" OFF)
 option(IREE_BUILD_EXPERIMENTAL_WEB_SAMPLES "Builds experimental web samples." OFF)
 
 #-------------------------------------------------------------------------------