testsuite: Handle empty assembly lines in check-function-bodies

I tried to make use of check-function-bodies for cris-elf and was a
bit surprised to see it failing.  There's a deliberate empty line
after the filled delay slot of the return-function which was
mishandled.  I thought "aha" and tried to add an empty line
(containing just a "**" prefix) to the match, but that didn't help.
While it was added as input from the function's assembly output
to-be-matched like any other line, it couldn't be matched: I had to
use "...", which works but is...distracting.

Some digging shows that an empty assembly line can't be deliberately
matched because all matcher lines (lines starting with the prefix,
the ubiquitous "**") are canonicalized by trimming leading
whitespace (the "string trim" in check-function-bodies) and instead
adding a leading TAB character, thus empty lines end up containing
just a TAB.  For usability it's better to treat empty lines as fluff
than to uglifying the test-case and the code to properly match them.
Double-checking, no test-case tries to match an line containing just
TAB (by providing an a line containing just "**\s*", i.e. zero or
more whitespace characters).

	* lib/scanasm.exp (parse_function_bodies): Set fluff to include
	empty lines (besides optionally leading whitespace).