VS: Populate `-Qspectre-` flag table entry for v143

Apply the change from commit 43aa632f57 (VS: Populate `-Qspectre-` flag
table entry for v142, 2019-01-24, v3.14.0-rc1~74^2~7) to the v143 flag
table.  Sort the longer options before the shorter ones.  Remove the
default variant with an empty switch.