perf genelf: Switch from SHA-1 to BLAKE2s for build ID generation

perf genelf: Switch from SHA-1 to BLAKE2s for build ID generation

Recent patches [1] [2] added an implementation of SHA-1 to perf and made
it be used for build ID generation.

I had understood the choice of SHA-1, which is a legacy algorithm, to be
for backwards compatibility.

It turns out, though, that there's no backwards compatibility
requirement here other than the size of the build ID field, which is
fixed at 20 bytes.  Not only did the hash algorithm already change (from
MD5 to SHA-1), but the inputs to the hash changed too: from 'load_addr
|| code' to just 'code', and now again to 'code || symtab || strsym'
[3].  Different linkers generate different build IDs, with the LLVM
linker using BLAKE3 hashes for example [4].

Therefore, we might as well switch to a more modern algorithm.  Let's go
with BLAKE2s.  It's faster than SHA-1, isn't cryptographically broken,
is easier to implement than BLAKE3, and the kernel's implementation in
lib/crypto/blake2s.c is easily borrowed.  It also natively supports
variable-length hashes, so it can directly produce the needed 20 bytes.

Also make the following additional improvements:

- Hash the three inputs incrementally, so they don't all have to be
  concatenated into one buffer.

- Add tag/length prefixes to each of the three inputs, so that distinct
  input tuples reliably result in distinct hashes.

[1] https://lore.kernel.org/linux-perf-users/20250521225307.743726-1-yuzhuo@google.com/
[2] https://lore.kernel.org/linux-perf-users/20250625202311.23244-1-ebiggers@kernel.org/
[3] https://lore.kernel.org/linux-perf-users/20251125080748.461014-1-namhyung@kernel.org/
[4] https://github.com/llvm/llvm-project/commit/d3e5b6f7539b86995aef6e2075c1edb3059385ce

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Tested-by: Ian Rogers <irogers@google.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Fangrui Song <maskray@sourceware.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Pablo Galindo <pablogsal@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>