[GDB/testsuite] Use %progbits in watch-loc.c

While using @progbits in .pushsection work on some targets, it does not
work on arm target where this introduces a comment. This patch replaces
its use in gdb.dlang/watch-loc.c and gdb.mi/dw2-ref-missing-frame-func.c
by %progbits which should work on all targets since it is used in
target-independent elf/section7.s GAS test.

2018-03-02  Thomas Preud'homme  <thomas.preudhomme@arm.com>

gdb/testsuite/
	* gdb.dlang/watch-loc.c: Use %progbits instead of @progbits.
	* gdb.mi/dw2-ref-missing-frame-func.c: Likewise.