Hide warning on static executables in strip.sh

Stripping static executables prints a warning:
bash: prebuilts/gcc/linux-x86/arm/arm-linux-androideabi-4.9/bin/arm-linux-androideabi-nm: No such file or directory

The message is harmless, just hide it.

Also turn on pipefail so that the script will exit if the command
returns an error.

Bug: 24409581
Test: m checkbuild
Change-Id: I9647c55c3509e4573e6d4f2f6d88119bdf31e9fb