docs/devel/build-system.rst: Format literals correctly

docs/devel/build-system.rst: Format literals correctly

In rST markup, single backticks `like this` represent "interpreted
text", which can be handled as a bunch of different things if tagged
with a specific "role":
https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#interpreted-text
(the most common one for us is "reference to a URL, which gets
hyperlinked").

The default "role" if none is specified is "title_reference",
intended for references to book or article titles, and it renders
into the HTML as <cite>...</cite> (usually comes out as italics).

build-system.rst seems to have been written under the mistaken
assumption that single-backticks mark up literal text (function
names, etc) which should be rendered in a fixed-width font.
The rST markup for this is ``double backticks``.

Update all the markup.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210726142338.31872-2-peter.maydell@linaro.org