DRTVWR-418, MAINT-6996: Update Mac LLMemory::getCurrentRSS().

Evidently the Mac implementation of LLMemory::getCurrentRSS() goes back to
OS X 10.3, because there was a helpful comment of the form:

------
The API used here is not capable of dealing with 64-bit memory sizes, but is
available before 10.4.

Once we start requiring 10.4, we can use the updated API, which looks like
this:

[new current implementation]

Of course, this doesn't gain us anything unless we start building the viewer
as a 64-bit executable, since that's the only way for our memory allocation to
exceed 2^32.
------

Hey, guess what, we're building 64-bit viewers now!

Thank you, whoever thoughtfully noted that, both for calling out the issue and
sparing us the research. (The comment goes back to Subversion days, so hg
blame shows only the merge-to-release changeset.)

indra/llcommon/llmemory.cpp

@@ -255,19 +255,6 @@ U64 LLMemory::getCurrentRSS()
 
 #elif defined(LL_DARWIN)
 
-/* 
-	The API used here is not capable of dealing with 64-bit memory sizes, but is available before 10.4.
-	
-	Once we start requiring 10.4, we can use the updated API, which looks like this:
-	
-	task_basic_info_64_data_t basicInfo;
-	mach_msg_type_number_t  basicInfoCount = TASK_BASIC_INFO_64_COUNT;
-	if (task_info(mach_task_self(), TASK_BASIC_INFO_64, (task_info_t)&basicInfo, &basicInfoCount) == KERN_SUCCESS)
-	
-	Of course, this doesn't gain us anything unless we start building the viewer as a 64-bit executable, since that's the only way
-	for our memory allocation to exceed 2^32.
-*/
-
 // 	if (sysctl(ctl, 2, &page_size, &size, NULL, 0) == -1)
 // 	{
 // 		LL_WARNS() << "Couldn't get page size" << LL_ENDL;
@@ -280,9 +267,9 @@ U64 LLMemory::getCurrentRSS()
 U64 LLMemory::getCurrentRSS()
 {
 	U64 residentSize = 0;
-	task_basic_info_data_t basicInfo;
-	mach_msg_type_number_t  basicInfoCount = TASK_BASIC_INFO_COUNT;
-	if (task_info(mach_task_self(), TASK_BASIC_INFO, (task_info_t)&basicInfo, &basicInfoCount) == KERN_SUCCESS)
+	task_basic_info_64_data_t basicInfo;
+	mach_msg_type_number_t  basicInfoCount = TASK_BASIC_INFO_64_COUNT;
+	if (task_info(mach_task_self(), TASK_BASIC_INFO_64, (task_info_t)&basicInfo, &basicInfoCount) == KERN_SUCCESS)
 	{
 		residentSize = basicInfo.resident_size;