fix: another attempt to avoid sigpipe on grep (#519)

Take this opportunity to factorize all the greps in /proc/cpuinfo
into a helper that avoids using a pipe to entirely avoid SIGPIPE
on a possibly gigantic /proc/cpuinfo

(cherry picked from commit 5bbffaf053)

src/vulns/CVE-2017-5715.sh

@@ -306,7 +306,7 @@ check_CVE_2017_5715_linux() {
                 # which in that case means ibrs is supported *and* enabled for kernel & user
                 # as per the ibrs patch series v3
                 if [ -z "$g_ibrs_supported" ]; then
-                    if grep ^flags "$g_procfs/cpuinfo" | grep -qw spec_ctrl_ibrs; then
+                    if cpuinfo_has_flag spec_ctrl_ibrs; then
                         pr_debug "ibrs: found spec_ctrl_ibrs flag in $g_procfs/cpuinfo"
                         g_ibrs_supported="spec_ctrl_ibrs flag in $g_procfs/cpuinfo"
                         # enabled=2 -> kernel & user