Prevent repeated ASLR reexec attempts

[Haihan-Jiang]

What changed

• Add an internal BENCHMARK_ASLR_REEXEC_ATTEMPTED sentinel before MaybeReenterWithoutASLR() calls execv().
• If the benchmark process is re-entered but the no-ASLR personality was reset by a security policy, the next process sees the sentinel and skips another re-exec attempt.
• Add a gtest covering the sentinel short-circuit path.

Why

This prevents an infinite re-exec loop when personality() appears to set ADDR_NO_RANDOMIZE in the current process, but AppArmor resets that personality across execv().

Fixes #2184.

Validation

• git diff --check
• bazel test //test:benchmark_gtest --cxxopt=-isystem --cxxopt=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1

Note: the explicit --cxxopt is needed on my macOS local environment because the default Command Line Tools compiler is not finding libc++ headers without the SDK include path.

[LebedevRI]

Duplicate of #2193.