[gdb/testsuite] Fix gdb.tui/tui-layout-asm-short-prog.S compilation

[gdb/testsuite] Fix gdb.tui/tui-layout-asm-short-prog.S compilation

The test-case gdb.tui/tui-layout-asm-short-prog.exp uses an assembly file
gdb.tui/tui-layout-asm-short-prog.S that it compiles using -nostdlib and
-nostartfiles.

However, on x86_64-linux the resulting executable still has dependencies on
libm and libc:
...
$ ldd outputs/gdb.tui/tui-layout-asm-short-prog/tui-layout-asm-short-prog
	linux-vdso.so.1 (0x00007ffddf3ec000)
	libm.so.6 => /lib64/libm.so.6 (0x00007f8b13406000)
	libc.so.6 => /lib64/libc.so.6 (0x00007f8b13000000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f8b1350f000)
...
due -lm being added by default_target_compile.

On x86_64-freebsd, using -nostdlib and -nostartfiles in combination with -lm
causes the compilation to fail.

Fix this by using -static as well.

Tested on x86_64-linux and x86_64-freebsd.