builtins: tag with noexecstack

These routines do not require executable stacks.  However, by default ELFish
linkers may assume an executable stack on GNUish environments (and some non-GNU
ones too!).  The GNU extension to add a note to indicate a non-executable stack
is honoured by these environments to mark the stack as non-executable (the
compiler normally emits this directive on appropriate targets whenever
possible).  This allows normal builds from getting executable stacks due to
linking to the compiler rt builtins.

llvm-svn: 273500