a/drv/stm32f4/uart: enable LTO build.
a/drv/stm32f4/uart: enable LTO build.

This reduces the size of the driver binary by 40%, such that it is now
well under a kibibyte.

cbiffle/brittle-kernel,cbiffle/brittle-kernel,cbiffle/brittle-kernel,cbiffle/brittle-kernel