Add -j$(nproc) by default for make linux

After forgetfully running make without -j a couple of times,
fixing it once and for all.

Test: make runs faster, yay!
Change-Id: I9b62b1410a3b9261e62fc078ff8eed85bd02818a
diff --git a/Makefile b/Makefile
index 8dbe270..ca0dd53 100644
--- a/Makefile
+++ b/Makefile
@@ -14,6 +14,9 @@
 
 SELF := $(abspath $(lastword $(MAKEFILE_LIST)))
 
+CPUS ?= $(shell nproc)
+MAKEFLAGS += --jobs=$(CPUS)
+
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