testsuite: arm: Add pattern for armv8-m.base to cmse-15.c test

Since armv8-m.base uses thumb1 that does not suport sibcall/tailcall,
a pattern is needed that uses PUSH/BL/POP sequence instead of a single
B instruction to reuse an already existing function in the compile unit.

gcc/testsuite/ChangeLog:

	* gcc.target/arm/cmse/cmse-15.c: Added pattern for armv8-m.base.

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