[asan] Disable asm instrumentation on unsupported platforms.

Only emit calls to compiler-rt asm routines on platforms where they are
present (currently limited to linux i386/x86_64).

Patch by Yuri Gorshenin.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207651 91177308-0d34-0410-b5e6-96231b3b80d8