Fix calls in gdb.arch/thumb2-it.exp

Fix calls in gdb.arch/thumb2-it.exp

Tests in gdb.arch/thumb2-it.exp call functions defined in assembly
without type debugging information. Since
7022349d5c86bae74b49225515f42d2e221bd368 ("Stop assuming no-debug-info
functions return int") this triggers an error which leads to many tests
to FAIL. This patch cast the call to indicate the return type of the
functions when calling them.

2017-09-06  Thomas Preud'homme  <thomas.preudhomme@arm.com>

gdb/testsuite/
	* gdb.arch/thumb2-it.exp: Cast call to assembly defined function.