[libc++] Performance differences between libc++ and libstdc++ in cpu2017 xalan benchmark

[shivaramaarao]

When trying to use libc++ with cpu2017 xalan benchmark, seeing following performance issue

machine: amd znver5

compiler used: llvm 21
options used: -O3 -march=znver5 -flto

single copy runtime with libc++ : 143

single copy runtime with libstdc++ : 131

There seem to be 9% difference

perf report shows that more time is spent in std::find function. when libstdc++ is used, the function is vectorized and a scalar variant is used with libc++.

[frederick-vs-ja]

Seems already tracked in #84663.