dde_linux: implement memory barriers in USB driver

This patch implements the memory barrier macros in the USB driver for ARMv7
and x86.

Fixes #1159.
ram_blk: change blocksize output to decimal

Reading 200 instead of 512 is confusing.

Fixes #1142.
ram_blk: remove superfluous block_number check

block_number is Block::sector_t (uint64_t), which never can by less
than 0.

Fixes #1142.
Qt5: use the Genode main thread

Now that it is possible to resize the stack of the Genode main thread, it
is not necessary anymore to create a new Genode thread as Qt main thread.

Fixes #1134.
Qt5: constrain the window buffer refresh area

With this patch, the information about the changed window area is taken
into account when refreshing the Nitpicker buffer.

Fixes #1132.