Fix missing CHAIN_SUCCESS_COUNTER symbol in libcoreclr.so

Certain versions of the clang toolchain build libcoreclr.so with this symbol undefined. This symbol is just an alias of g_dispatch_cache_chain_success_counter and the issue was probably caused by the fact that we pass the CHAIN_SUCCESS_COUNTER as a parameter to macro and that macro then appends the @GOTPCREL to the symbol. It seems that in the problematic case, the assembler appends the @GOTPCREL to the CHAIN_SUCCESS_COUNTER instead of the original g_dispatch_cache_chain_success_counter symbol.
Changing the
CHAIN_SUCCESS_COUNTER = g_dispatch_cache_chain_success_counter
to
#define CHAIN_SUCCESS_COUNTER g_dispatch_cache_chain_success_counter
fixes the issue.

[tfs-changeset: 1411600]

Author: janvorli

src/vm/amd64/virtualcallstubamd64.S

@@ -10,7 +10,7 @@
 // entry is promoted to the front of the chain. This is declared as extern because
 // the default value (CALL_STUB_CACHE_INITIAL_SUCCESS_COUNT) is defined in the header.
 // extern size_t g_dispatch_cache_chain_success_counter;
-CHAIN_SUCCESS_COUNTER = g_dispatch_cache_chain_success_counter
+#define CHAIN_SUCCESS_COUNTER g_dispatch_cache_chain_success_counter
 
 // The reason for not using .equ or '=' here is that otherwise the assembler compiles e.g. 
 // mov rax, BACKPATCH_FLAG as mov rax, [BACKPATCH_FLAG]