[scons] Enable `__attribute__((packed))` on MinGW.
[scons] Enable `__attribute__((packed))` on MinGW.

By default MinGW ignores packed structs, but xpcc requires it for its
communication packets. `-mno-ms-bitfields` forces the compiler to
respect packed structs.
dergraaf/xpcc,dergraaf/xpcc,dergraaf/xpcc,dergraaf/xpcc