Load async-profiler.jar if possible when using -Xprofile

Instead of requiring it to be on the compiler classpath.

This will make it much easier to profile the Kotlin compiler daemon in
Gradle, by just specifying a compiler argument instead of also manually
patching the compiler jar.

compiler/testData/cli/jvm/extraHelp.out

@@ -100,9 +100,9 @@ where advanced options include:
                              0 means use a thread per processor core.
                              Default value is 1
   -Xprofile=<profilerPath:command:outputDir>
-                             Debug option: Run compiler with async profiler, save snapshots to outputDir, command is passed to async-profiler on start
-                             You'll have to provide async-profiler.jar on classpath to use this
-                             profilerPath is a path to libasyncProfiler.so
+                             Debug option: Run compiler with async profiler and save snapshots to `outputDir`; `command` is passed to async-profiler on start.
+                             `profilerPath` is a path to libasyncProfiler.so; async-profiler.jar should be on the compiler classpath.
+                             If it's not on the classpath, the compiler will attempt to load async-profiler.jar from the containing directory of profilerPath.
                              Example: -Xprofile=<PATH_TO_ASYNC_PROFILER>/async-profiler/build/libasyncProfiler.so:event=cpu,interval=1ms,threads,start,framebuf=50000000:<SNAPSHOT_DIR_PATH>
   -Xrepeat=<number>          Debug option: Repeats modules compilation <number> times
   -Xsam-conversions={class|indy} Select code generation scheme for SAM conversions.