Handle an unknown binary platform type in debugserver

debugserver parses the Mach-O header & load commands of
binaries; if it does this with a binary whose LC_BUILD
platform enum it does not recognize, it will currently crash.
This patch changes MachProcss::GetPlatformString to return
an optional platform string, and updates the callers to
do the right thing when this optional could not be
provided.

Differential Revision: https://reviews.llvm.org/D136719
rdar://100452994