riscv64: minor ASLR bits changes.

Improve the comment, and increase the "minimum maximum" to 24 --- we
only support 64-bit, and 64-bit never has less than 24 bits.

Bug: https://github.com/google/android-riscv64/issues/1
Test: treehugger
Change-Id: I478c7649aa19258352c908a449cabe12da94952c