i386: Replace assembly versions of e_powf with generic e_powf.c

This patch replaces i386 assembly versions of e_powf with generic
e_powf.c.  For workload-spec2017.wrf, on Nehalem, it improves
performance by:

                           Before            After     Improvement
reciprocal-throughput      230.855          78.3358       194%
latency                    231.685          94.1259       146%

On Skylake, it improves performance by:

                           Before            After     Improvement
reciprocal-throughput      239.858          47.4713       405%
latency                    247.57           93.8798       163%

On IvyBridge with --disable-multi-arch, it improves performance by:

                           Before            After     Improvement
reciprocal-throughput      269.078          63.3758       324%
latency                    271.473          102.091       165%

	* sysdeps/i386/fpu/e_powf.S: Removed.
	* sysdeps/i386/fpu/e_powf_log2_data.c: Likewise.
	* sysdeps/i386/fpu/w_powf.c: Likewise.
	* sysdeps/i386/fpu/libm-test-ulps: Updated for generic e_powf.c.
	* sysdeps/i386/i686/fpu/multiarch/libm-test-ulps: Likewise.
	* sysdeps/i386/i686/fpu/multiarch/Makefile (libm-sysdep_routines):
	Add e_powf-sse2.
	(CFLAGS-e_powf-sse2.c): New.
	* sysdeps/i386/i686/fpu/multiarch/e_powf-sse2.c: New file.
	* sysdeps/i386/i686/fpu/multiarch/e_powf.c: Likewise.