Simplify the breakpoint setting in DynamicLoaderMacOS::SetNotificationBreakpoint

Instead of looking up a symbol and reducing it to an addr_t to set
a breakpoint, set the breakpoint on the function name directly.
The old Mac OS X dynamic loader plugin worked in terms of addresses
and I incorrectly emulated that here when I wrote this newer one.

Differential Revision: https://reviews.llvm.org/D100931