ld: Print 0 size in B and not in GB

When using --print-memory-usage, the printed size can be zero and in
that case, the unit should be B and not GB.

ld/
	* ldlang.c (lang_print_memory_size) Print 0 B instead of 0 GB.
	* testsuite/ld-scripts/print-memory-usage-1.l: Validate emplty region.
	* testsuite/ld-scripts/print-memory-usage-1.t: Define empty region.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>