Replace call stack keyword with path to runtime directory

[VincentBu]

Since we don't know the call stack of given dump ahead of time, it doesn't make sense to set 'callStackKeyword' before running the analyzer script.
A workaround for this is: searching for the first line which contains path of runtime.(It's likely to be some method in runtime or ReliabilityFramework).
For instance, the line 3

00 (Inline Function) --------`--------     KERNELBASE!DebugBreak [minkernel\kernelbase\debug.c @ 143] 
01 00000051`10ddd8a8 00007ffb`37b068b0     KERNELBASE!wil::details::DebugBreak+0x2 [onecore\internal\sdk\inc\wil\opensource\wil\result_macros.h @ 1836] 
02 00000051`10ddd8b0 00000051`10ddd8e8     ReliabilityFramework!ReliabilityFramework.MyDebugBreak+0xa0 [Q:\runtime\src\tests\GC\Stress\Framework\ReliabilityFramework.cs @ 541] 
03 00000051`10ddd8b8 000001c8`77b621f0     0x00000051`10ddd8e8
04 00000051`10ddd8c0 00007ffb`95b21ed8     0x000001c8`77b621f0
05 00000051`10ddd8c8 00007ffb`95b21bd8     coreclr!`string'
06 00000051`10ddd8d0 00000051`10ddd8b0     coreclr!`string'
07 00000051`10ddd8d8 00007ffb`37655357     0x00000051`10ddd8b0
08 00000051`10ddd8e0 9abcdef0`12345678     ReliabilityFramework!RFLogging.WriteToInstrumentationLog(ReliabilityTestSet, LoggingLevels, System.String)+0x16dbc47
09 00000051`10ddd8e8 00007ffb`95afbff8     0x9abcdef0`12345678
0a 00000051`10ddd8f0 ffffffff`ffffffff     coreclr!vtable_InlinedCallFrame
0b 00000051`10ddd8f8 00007ffb`361e19a0     0xffffffff`ffffffff
0c 00000051`10ddd900 00000051`10ddd8b0     0x00007ffb`361e19a0
0d 00000051`10ddd908 00007ffb`37b068b0     0x00000051`10ddd8b0
0e 00000051`10ddd910 00000051`10ddd960     ReliabilityFramework!ReliabilityFramework.MyDebugBreak+0xa0 [Q:\runtime\src\tests\GC\Stress\Framework\ReliabilityFramework.cs @ 541] 
0f 00000051`10ddd918 00000000`00000000     0x00000051`10ddd960

[mrsharm]

This should be changed back to looking at the ::gc_heap:: and then we should handle all the call stacks that don't contain "::gc_heap::" as unknown by aggregating them in the "Unknown" item in the dictionary.