[XRay][AArch64] Suppport __xray_customevent/__xray_typedevent

`__xray_customevent` and `__xray_typedevent` are built-in functions in Clang.
With -fxray-instrument, they are lowered to intrinsics llvm.xray.customevent and
llvm.xray.typedevent, respectively. These intrinsics are then lowered to
TargetOpcode::{PATCHABLE_EVENT_CALL,PATCHABLE_TYPED_EVENT_CALL}. The target is
responsible for generating a code sequence that calls either
`__xray_CustomEvent` (with 2 arguments) or `__xray_TypedEvent` (with 3
arguments).

Before patching, the code sequence is prefixed by a branch instruction that
skips the rest of the code sequence. After patching
(compiler-rt/lib/xray/xray_AArch64.cpp), the branch instruction becomes a NOP
and the function call will take effects.

This patch implements the lowering process for
{PATCHABLE_EVENT_CALL,PATCHABLE_TYPED_EVENT_CALL} and implements the runtime.

```
// Lowering of PATCHABLE_EVENT_CALL
.Lxray_sled_N:
  b  #24
  stp x0, x1, [sp, #-16]!
  x0 = reg of op0
  x1 = reg of op1
  bl __xray_CustomEvent
  ldrp x0, x1, [sp], #16
```

As a result, two updated tests in compiler-rt/test/xray/TestCases/Posix/ now
pass on AArch64.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D153320