cpu: add zero_denormals method

Add a method to enable/disable the denormals flush-to-zero and
denormals-as-zero CPU options.

Add a config option to make it possible to disable this again.

Fixes high CPU usage when dealing with denormals, which can happen
in many DSP functions.

Fixes #1681

src/daemon/pipewire.conf.in

@@ -15,6 +15,7 @@ context.properties = {
     #mem.mlock-all                         = false
     #clock.power-of-two-quantum            = true
     #log.level                             = 2
+    #cpu.zero.denormals                    = true
 
     core.daemon = true              # listening for socket connections
     core.name   = pipewire-0        # core name and socket name