[roll] Roll fuchsia [kernel] Template SpscBuffer on an allocator

Prior to this CL, the SpscBuffer relied on the caller to provide
a buffer to use as its backing storage_. This led to some convoluted
lifetime management, as the SpscBuffer was not aware if the
underlying storage was destroyed.

This CL fixes the issue by templating the SpscBuffer on an Allocator
type, which it then uses to:
1. Allocate storage_ during Init, and
2. Free storage_ during destruction of the SpscBuffer.

Original-Bug: 404539312
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1248025
Original-Revision: 4ce44ec1f18f6b7e764051e88f8cbf929b3a3cac
GitOrigin-RevId: 5d250a4094c078e86ec2a9011a2562c2aa706a37
Change-Id: Ia85f7e5d20609eb552519f9ab643b2272c2c567f
diff --git a/stem b/stem
index 2a5650a..bc8a74c 100644
--- a/stem
+++ b/stem
@@ -11,7 +11,7 @@
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              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="8ffaf016854ad273068754d6c68916c32ebd870b"
+             revision="4ce44ec1f18f6b7e764051e88f8cbf929b3a3cac"
              />
   </projects>
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