[LTO] Scan all per-function subtargets when collecting runtime library names.

accumulateAndSortLibcalls in LTOCodeGenerator.cpp collects names of runtime
library functions which are used to identify user-defined functions that should
be protected. Previously, this function would only scan the TargetLowering
object belonging to the "main" subtarget for the library function names. This
commit changes it to scan all per-function subtargets.

Differential Revision: http://reviews.llvm.org/D7275

llvm-svn: 227533