ARM64: Enable Tail Call with Vararg

Fixes https://github.com/dotnet/coreclr/issues/4475
I've run into `IMPL_LIMITATION("varags + CEE_JMP doesn't work yet")` in
importer.cpp. This change enables ARM64 tail call path same as other
targets.
 1. Similar to amd64 `genFnEpilog`, I made the similar code under
 `!FEATURE_FASTTAILCALL`. Since `EC_FUNC_TOKEN_INDIR` is not defined for
 arm64, I've made NYI for such case.
 2. Added two pseudo branch instructions 'b_tail' and 'br_tail' which form
 jmp instruction encodings but follow call instruction semantics since
 they are used for tail-call.
 3. `GenJmpMethod` is enabled. Code is slightly changed to reflect correct
 float argument handlings and multi-reg support.

1 files changed, 2 insertions, 5 deletions

diff --git a/src/jit/lowerarm64.cpp b/src/jit/lowerarm64.cpp
index 5c53e253c8..82a6e3c12e 100644
--- a/src/jit/lowerarm64.cpp
+++ b/src/jit/lowerarm64.cpp
@@ -722,17 +722,14 @@ void Lowering::TreeNodeInfoInit(GenTree* stmt)
 
                 // If it is a fast tail call, it is already preferenced to use IP0.
                 // Therefore, no need set src candidates on call tgt again.
-                if (tree->gtCall.IsVarargs() && 
-                    callHasFloatRegArgs &&                 
+                if (tree->gtCall.IsVarargs() &&
+                    callHasFloatRegArgs &&
                     !tree->gtCall.IsFastTailCall() &&
                     (ctrlExpr != nullptr))
                 {
                     // Don't assign the call target to any of the argument registers because
                     // we will use them to also pass floating point arguments as required
                     // by Arm64 ABI.
-                    
-                    NYI_ARM64("Lower - IsVarargs");
-
                     ctrlExpr->gtLsraInfo.setSrcCandidates(l, l->allRegs(TYP_INT) & ~(RBM_ARG_REGS));
                 }
             }