Port async-profiler#1701: optimize nmethod type checks

[jbachorik]

What does this PR do?:
Ports async-profiler#1701 (commit cc0eab1) into the Datadog fork, adapting it for our VMNMethod-based class structure.

Motivation:
Upstream measured up to ~11% improvement in average stack-walking speed on real benchmarks (spring-petclinic, renaissance dotty). The two main micro-optimizations are:

1. isInterpreter() pointer comparison — HotSpot has exactly one Interpreter code blob. We now record its VMNMethod* at JVM startup (via addRuntimeStub) and compare with this instead of calling strcmp. O(1) instead of O(n).
2. Inline startsWith template — Replaces strcmp/strncmp calls in isNMethod, isStub, and isVTableStub with a compile-time-unrolled character loop, avoiding the indirect PLT call to the standard library.
3. Branch reordering in stackWalker.cpp — isInterpreter() is now checked before isNMethod() since it is now a single pointer comparison.

Additional Notes:

• _interpreter_start is stored separately so that resolveOffsets() can retry findNMethod if the code heap was not yet initialized when addRuntimeStub("Interpreter", ...) was called.
• The improvement can be observed via the existing WALKVM_* counters (e.g. walkvm_java_frame_ok) recorded during profiling runs.
• Closes Upstream async-profiler changes detected (2026-03-19) #421.

How to test the change?:
Existing CI covers the stack-walking paths. The performance gain can be measured by comparing stack-walk throughput on a CPU-heavy workload before and after.

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.
• JIRA: [JIRA-XXXX]

Unsure? Have a question? Request a review!

[dd-octo-sts]

CI Test Results

Run: #23315546091 | Commit: 03046c0 | Duration: 10m 2s (longest job)

✅ All 32 test jobs passed

Status Overview

JDK	glibc-aarch64/debug	glibc-amd64/debug	musl-aarch64/debug	musl-amd64/debug
8	-	✅	-	-
8-ibm	-	✅	-	-
8-j9	✅	✅	-	-
8-librca	-	-	✅	✅
8-orcl	-	✅	-	-
11	-	✅	-	-
11-j9	✅	✅	-	-
11-librca	-	-	✅	✅
17	✅	✅	-	-
17-graal	✅	✅	-	-
17-j9	✅	✅	-	-
17-librca	-	-	✅	✅
21	✅	✅	-	-
21-graal	✅	✅	-	-
21-librca	-	-	✅	✅
25	✅	✅	-	-
25-graal	✅	✅	-	-
25-librca	-	-	✅	✅

Legend: ✅ passed | ❌ failed | ⚪ skipped | 🚫 cancelled

Summary: Total: 32 | Passed: 32 | Failed: 0

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Updated: 2026-03-19 20:44:23 UTC