Summary
-------

Introduce JVMTI feature to support tracking total GC CPU time.

Problem
-------

Users are interested in tracking GC CPU time with JVMTI agents.

Solution
--------

Introduce new JVMTI API's:

 - new capability: can_get_gc_cpu_time
 - new JVMTI functions: GetGCCpuTimerInfo, GetTotalGCCpuTime

Specification
-------------

New JVMTI API's include:

 - new capability: can_get_gc_cpu_time
 - new functions:
     - jvmtiError GetGCCpuTimerInfo(jvmtiEnv* env, jvmtiTimerInfo* info_ptr)
     - jvmtiError GetTotalGCCpuTime(jvmtiEnv* env, jlong* nanos_ptr)

The JVMTI spec update is:

     <TBD: To add JVMTI spec-diff>


The `src/hotspot/share/prims/jvmti.xml` diff is:

    @@ -10520,10 +10520,16 @@
             <description>
               Can <functionlink id="GetThreadCpuTime">get</functionlink>
               thread CPU time
             </description>
           </capabilityfield>
    +      <capabilityfield id="can_get_gc_cpu_time" since="26">
    +        <description>
    +          Can <functionlink id="GetTotalGCCpuTime">get</functionlink>
    +          GC CPU time
    +        </description>
    +      </capabilityfield>
           <capabilityfield id="can_generate_method_entry_events"
                            disp1="can_generate" disp2="_method_entry_events"
                            >
             <description>
               Can generate method entry events on entering a method
    @@ -11223,10 +11229,72 @@
           </parameters>
           <errors>
           </errors>
         </function>
     
    +    <function id="GetGCCpuTimerInfo" phase="any" callbacksafe="safe" num="157" since="26">
    +      <synopsis>Get Total GC Cpu Timer Information</synopsis>
    +      <description>
    +        Get information about the
    +        <functionlink id="GetTotalGCCpuTime"/> timer.
    +        The fields of the <datalink id="jvmtiTimerInfo"/> structure
    +        are filled in with details about the timer.
    +        This information will not change during a particular invocation of the VM.
    +      </description>
    +      <origin>new</origin>
    +      <capabilities>
    +        <required id="can_get_gc_cpu_time">
    +            Can get GC cpu time.
    +        </required>
    +      </capabilities>
    +      <parameters>
    +        <param id="info_ptr">
    +          <outptr><struct>jvmtiTimerInfo</struct></outptr>
    +          <description>
    +            On return, filled with information describing the time
    +            returned by <functionlink id="GetTotalGCCpuTime"/>.
    +          </description>
    +        </param>
    +      </parameters>
    +      <errors>
    +      </errors>
    +    </function>
    +
    +    <function id="GetTotalGCCpuTime" phase="any" callbacksafe="safe" num="158" since="26">
    +      <synopsis>Get Total GC CPU Time</synopsis>
    +      <description>
    +          Return the current value of the total GC CPU timer, in nanoseconds.
    +          <p/>
    +          The value returned represents nanoseconds since some fixed but
    +          arbitrary time (perhaps in the future, so values may be
    +          negative).  This function provides nanosecond precision, but not
    +          necessarily nanosecond accuracy. No guarantees are made about
    +          how frequently values change.
    +          <p/>
    +          Get information about this timer with
    +          <functionlink id="GetGCCpuTimerInfo"/>.
    +      </description>
    +      <origin>new</origin>
    +      <capabilities>
    +        <required id="can_get_gc_cpu_time">
    +            Can get GC cpu time.
    +        </required>
    +      </capabilities>
    +      <parameters>
    +        <param id="nanos_ptr">
    +          <outptr><jlong/></outptr>
    +          <description>
    +            On return, points to the time in nanoseconds.
    +            This is an unsigned value.  If tested or printed as a jlong (signed value)
    +            it may appear to be a negative number.
    +          </description>
    +        </param>
    +      </parameters>
    +      <errors>
    +      </errors>
    +    </function>
    +