[gdb/testsuite] Fix gdb.tui/tui-layout-asm-short-prog.exp on cygwin

[gdb/testsuite] Fix gdb.tui/tui-layout-asm-short-prog.exp on cygwin

On x86_64-cygwin, I run into:
...
 Running tui-layout-asm-short-prog.exp ...
 gdb compile failed, tui-layout-asm-short-prog.S: Assembler messages:
 tui-layout-asm-short-prog.S:26: Error: \
   junk at end of line, first unrecognized character is `-'

                 === gdb Summary ===

 # of untested testcases         1
...

This is due to this note in gdb.tui/tui-layout-asm-short-prog.S:
...
        .section        .note.GNU-stack,"",@progbits
...

The note can be easily skipped for cygwin, but after doing so, we run into a
fail.

The problem is that while the test-case tries to generate 5 nops:
...
 	.rept 5
 	nop
 	.endr
...
in fact 16 nops are generated.

The fail is not specific to x86_64-cygwin, if we do ".rept 16" on
x86_64-linux, we get the same fail.

Fix this by increasing the "large" scroll amounts from 13 to 15.

Tested on x86_64-cygwin and x86_64-linux.