libstdc++: Restore unrolling in std::find using pragma [PR116140]

libstdc++: Restore unrolling in std::find using pragma [PR116140]

Together with the preparatory compiler patches, this patch restores
unrolling in std::__find_if, but this time relying on the compiler to do
it by using:

  #pragma GCC unroll 4

which should restore the majority of the regression relative to the
hand-unrolled version while still being vectorizable with WIP alignment
peeling enhancements.

On Neoverse V1 with LTO, this reduces the regression in xalancbmk (from
SPEC CPU 2017) from 5.8% to 1.7% (restoring ~71% of the lost
performance).

libstdc++-v3/ChangeLog:

	PR libstdc++/116140
	* include/bits/stl_algobase.h (std::__find_if): Add #pragma to
	request GCC to unroll the loop.