use libc's powerof2

Instead of rolling our own powerof2 function, use the version provided
by libc. In particular, this version is not ubsan safe, whereas the libc
version is. See https://android-review.googlesource.com/c/platform/bionic/+/932279

Test: compiles and boots
Change-Id: I0dd39254357826c14c3185a5e6332faa90975f01