rtld: remove hand rolled memset and bzero

They were introduced to take care of ifunc, but right now no architecture
provides ifunc'ed variants. Since rtld uses memset extensively this results in
a pessmization. Should someone want to use ifunc here they should provide a
mandatory symbol (e.g., rtld_memset).

See the review for profiling data.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D23176