[gdb/testsuite] Fix gdb.arch/amd64-disp-step-avx.exp on x86_64-freebsd

[gdb/testsuite] Fix gdb.arch/amd64-disp-step-avx.exp on x86_64-freebsd

With test-case gdb.arch/amd64-disp-step-avx.exp on x86_64-freebsd I run into:
...
(gdb) continue
Continuing.

Breakpoint 3, test_rip_vex2_end () at amd64-disp-step-avx.S:35
35		nop
(gdb) FAIL: $exp: vex2: continue to test_rip_vex2_end
...

This happens while executing this bit of the test-case:
...
    # Turn "debug displaced" on to make sure a displaced step is actually
    # executed, not an inline step.
    gdb_test_no_output "set debug displaced on"

    gdb_test "continue" \
	"Continuing.*prepared successfully .*Breakpoint.*, ${test_end_label} ().*" \
	"continue to ${test_end_label}"
...

The problem is that on x86_64, displaced stepping is only supported for linux.
Consequently, the "prepared successfully" message is missing.

Fix this by requiring linux.

Approved-by: Kevin Buettner <kevinb@redhat.com>
Tested on x86_64-freebsd.