fix arm64 build

arm64 doesn't have __NR__sysctl defined because the sysctl
syscall is deprecated.  Since the vulnerability being tested
for only applies to arm, wrap the entire test in #ifdef __arm__.

(cherrypicked from f1105808)

Bug: 15408470
Change-Id: I2bef16aca21f5d9757ff02ee92ae86c9157615c9