test: add libc debuglink/build-id fixtures

libc.so.6 and libc.so.6.debug are the stripped libc plus its separate
debug file from Ubuntu 22.04 (jammy) glibc 2.35-0ubuntu3, amd64.

Extracted from the official .deb packages:

  pool/main/g/glibc/libc6_2.35-0ubuntu3_amd64.deb
  pool/main/g/glibc/libc6-dbg_2.35-0ubuntu3_amd64.deb

via `ar x` + `tar --zstd -xf data.tar.zst`. The debug file originally
lives under /usr/lib/debug/.build-id/89/c3cb85...1969.debug; it is
renamed to libc.so.6.debug in testdata/ for readability. Tests that
need the .gnu_debuglink basename stage a tempdir copy with the expected
name.

Build-id: 89c3cb85f9e55046776471fed05ec441581d1969

These fixtures exercise the build-id and .gnu_debuglink resolution
paths against a real-world stripped system library, complementing the
synthetic cpp_my_benchmark pair.

Author: not-matthias

testdata/perf_map/libc.so.6

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:ebaed8bd64291875c908cc6e7f5115ae7bf605a5da56ce3da4028e2fcfb0d8e5
+size 2216304

testdata/perf_map/libc.so.6.debug

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:f84c93df1dffba7b504b2bc3e5dd7996b87e734053f45f4ee1ad2fe266c2fa99
+size 4418528