Refactor and generalize AArch64 watchpoint support in debugserver

Refactor the debugserver watchpiont support in anticipating of
adding support for AArch64 MASK hardware watchpoints to watch
larger regions of memory.  debugserver already had support for
handling a request to watch an unaligned region of memory up
to 8 bytes using Byte Address Select watchpoints - it would split
an unaligned watch request into two aligned doublewords that
could be watched with two hardware watchpoints using the BAS
specification.

This patch generalizes that code for properly aligning, and
possibly splitting, a watchpoint request into two hardware watchpoints
to handle any size request.  And separates out the specifics
about BAS watchpoints into its own method, so a sibling method
for MASK watchpoints can be dropped in next.

Differential Revision: https://reviews.llvm.org/D149040
rdar://108233371