[ifunc] Use c implementation to compare cpu variant names

Did this for fun but turns out it generates better assembly code.

With a C implementation, compiler can expand both the comparison
loop and cpu_variant_names array. Results in a decision tree like
structure. For example the "cortex-a" prefix common in many variants
will only be compared once.

Test: run bionic unit tests.
Test: use debugger to check which variant is used.
Change-Id: Id70e5536f968161f20b18f35d46430457c338f0f