feat: target=x86_64_v2 minimum for (some) vectorization

• Intel Nehalem is x86_64_v2, launched 2008.
• Intel Haswell is x86_64_v3, launched 2013.

x86-64-v2 (circa 2009: Nehalem and Jaguar)

  CMPXCHG16B	cmpxchg16b
  LAHF-SAHF	lahf
  POPCNT	popcnt
  SSE3		addsubpd
  SSE4_1	blendpd
  SSE4_2	pcmpestri
  SSSE3		phaddd

x86-64-v3 (circa 2015: Haswell and Excavator)

  AVX		vzeroall
  AVX2		vpermd
  BMI1		andn
  BMI2		bzhi
  F16C		vcvtph2ps
  FMA		vfmadd132pd
  LZCNT		lzcnt
  MOVBE		movbe
  OSXSAVE	xgetbv