[sanitizer] Fix vfork interceptor on i386 w/ dynamic runtime.

PLT calls on i386 expect GOT base address in %ebx.
This call does not need to go through PLT strictly speaking, but I'd
rather avoid future surprises and implement the most general case.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@355125 91177308-0d34-0410-b5e6-96231b3b80d8