tcg/i386: Use host/cpuinfo.h

Use the CPUINFO_* bits instead of the individual boolean
variables that we had been using.  Remove all of the init
code that was moved over to cpuinfo-i386.c.

Note that have_avx512* check both AVX512{F,VL}, as we had
previously done during tcg_target_init.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>