[gdb/testsuite] Use more %progbits for arm

On pinebook I ran into:
...
Running gdb.tui/tui-layout-asm-short-prog.exp ...
gdb compile failed, gdb.tui/tui-layout-asm-short-prog.S: Assembler messages:
gdb.tui/tui-layout-asm-short-prog.S:23: Error: \
  junk at end of line, first unrecognized character is `,'
...

Fix this by using %progbits instead of @progbits for arm.

Approved-by: Luis Machado <luis.machado@arm.com>

Tested on x86_64-linux and pinebook.