[lldb/debugserver] Implement hardware breakpoints for x86_64 and i386

This implements hardware breakpoints for x86_64 and i386 in debugserver.
It's based on Pedro's patch sent to lldb-commits [1] although most of it
is the same as the existing hardware watchpoint implementation.

[1] http://lists.llvm.org/pipermail/lldb-commits/Week-of-Mon-20200113/060327.html

Differential revision: https://reviews.llvm.org/D72985