Fix a bug where lldb does not respect the packet size.

Summary: LLDB was using packet size advertised by the target as the max memory size to write in one go. It is wrong because packets have other overhead apart from memory payload. Also memory transferred through 'm' and 'M' packets needs 2 bytes in packet to transfer 1 of memory.

Reviewers: clayborg

Reviewed By: clayborg

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D28808

llvm-svn: 292987