Remove busybox modprobe to fix slow performance for some devices

bionic process initialization calls personality
(specifically personality-8)
personality wants to load a kernel module
loading a kernel module calls /sbin/modprobe
loading /sbin/modprobe is a bionic process initialization
bionic process initialization calls personality
personality wants to load a kernel module
loading a kernel module calls /sbin/modprobe . . .

Before you know it, it takes 0.5 seconds to do anything.
Note: modprobe is still technically available, but the symlink
has been removed, so you can still call it directly by running
busybox modprobe if you like.

From what I can tell, this issue only affects 32 bit devices
with CONFIG_MODULES=y in the defconfig. The problem can be also
patched out of the kernel by commenting or otherwise removing
the block of code in kernel/exec_domain.c inside the CONFIG_MODULES
ifdef block

It is also possible to patch the problem in bionic libc by commenting
out or otherwise removing the __initialize_personality in bionic/libc/
bionic/libc_init_common.cpp file.

Change-Id: Iebac314616080ac18320d73b087980ac1b98b951

Android.mk

@@ -466,6 +466,16 @@ include $(CLEAR_VARS)
 BUSYBOX_LINKS := $(shell cat external/busybox/busybox-full.links)
 exclude := tune2fs mke2fs mkdosfs mkfs.vfat gzip gunzip
 
+# Having /sbin/modprobe present on 32 bit devices with can cause a massive
+# performance problem if the kernel has CONFIG_MODULES=y
+ifeq ($(shell test $(PLATFORM_SDK_VERSION) -gt 22; echo $$?),0)
+    ifneq ($(TARGET_ARCH), arm64)
+        ifneq ($(TARGET_ARCH), x86_64)
+            exclude += modprobe
+        endif
+    endif
+endif
+
 # If busybox does not have restorecon, assume it does not have SELinux support.
 # Then, let toolbox provide 'ls' so -Z is available to list SELinux contexts.
 ifeq ($(TWHAVE_SELINUX), true)