Fix inconsistent target arch when attaching to arm64 binaries on

arm64e platforms.

On arm64e-capable Apple platforms, the system libraries are always
arm64e, but applications often are arm64. When a target is created
from file, LLDB recognizes it as an arm64 target, but debugserver will
still (technically correct) report the process as being arm64e. For
consistency, set the target to arm64 here.

rdar://92248684

Differential Revision: https://reviews.llvm.org/D133069