Dynamic "make -j" parameter

This obviously interferes with Bazel's own scheduler, but these are
our only compute-intensive compilation tasks, anyway.

Fixes T522

Test Plan:
{F16451}

(fun fact: that EPYC 7401P wasn't faster than the 3900X)

Bug: T522

X-Origin-Diff: phab/D236
GitOrigin-RevId: 8735ece9eea6ed2cd38fda8823a674d0298b6dbc
diff --git a/core/build/linux_kernel/BUILD b/core/build/linux_kernel/BUILD
index e130142..e1f9c9b 100644
--- a/core/build/linux_kernel/BUILD
+++ b/core/build/linux_kernel/BUILD
@@ -20,7 +20,7 @@
     cp $(locations @//core/build/utils) $$DIR/.bin/
     cp $(location initramfs.list) $$DIR/initramfs.list
 
-    (cd $$DIR && make -j 16) >/dev/null
+    (cd $$DIR && make -j $$(nproc)) >/dev/null
 
     cp $$DIR/arch/x86/boot/bzImage $(RULEDIR)
     """,