libstdc++: Improve "find_first/last_set" for NEON

libstdc++: Improve "find_first/last_set" for NEON

The find_first_set and find_last_set method is not optimal for neon, it
needs to be improved by synthesized with horizontal adds(vaddv) which
will reduce the generated assembly code. In the following cases,
vaddvq_s16 will generate 2 instructions but vpadd_s16 will generate 4
instructions:

 # vaddvq_s16
    vaddvq_s16(__asint);
    //  addv    h0, v1.8h
    //  smov    w1, v0.h[0]
 # vpadd_s16
    vpaddq_s16(vpaddq_s16(vpaddq_s16(__asint, __zero), __zero), __zero)[0]
    // addp v1.8h,v1.8h,v2.8h
    // addp v1.8h,v1.8h,v2.8h
    // addp v1.8h,v1.8h,v2.8h
    // smov    w1, v1.h[0]
 #

libstdc++-v3/ChangeLog:

	* include/experimental/bits/simd_neon.h: Replace repeated vpadd
	calls with a single vaddv for aarch64.