avutil/cpu: remove the |checked| static variable

Remove the |checked| variable because the invalid value of -1 for
|flags| can be used to indicate the same condition. Also rename |flags|
to |cpu_flags| because there are a local variable and a function
parameter named |flags| in the same file.

Co-author: Dmitry Vyukov of Google

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>