Change static buffer to be BSS instead of DATA in HandlePacket_qSpeedTest

Having this 4MB buffer with a compile-time initialized string forced it
into the DATA section and it took up 4MB of space in the binary, which
accounts for like 80% of debugserver's footprint on disk.  Change it to
BSS and strcpy in the initial value at runtime instead.

<rdar://problem/73503892>