minidump: Rename some architecture constants

The architecture enum contains two kinds of contstants: the "official" ones
defined by Microsoft, and unofficial constants added by breakpad to cover the
architectures not described by the first ones.

Up until now, there was no big need to differentiate between the two. However,
now that Microsoft has defined
https://docs.microsoft.com/en-us/windows/win32/api/sysinfoapi/ns-sysinfoapi-system_info
a constant for ARM64, we have a name clash.

This patch renames all breakpad-defined constants with to include the prefix
"BP_". This frees up the name "ARM64", which I'll re-introduce with the new
"official" value in a follow-up patch.

Reviewers: amccarth, clayborg

Subscribers: lldb-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D69285