[gdb/testsuite] Use support_displaced_stepping in gdb.arch/amd64-disp-step-avx.exp

In commit 8e73fddeb0d ("[gdb/testsuite] Fix gdb.arch/amd64-disp-step-avx.exp
on x86_64-freebsd") I added a "require {istarget *-*-linux*}", but since then
I found support_displaced_stepping, which seems more appropriate and
descriptive.

Fix this by requiring support_displaced_stepping instead.

Tested on x86_64-freebsd.
1 file changed