Allow ThreadLauncher::LaunchThread() to specify a minimum stack byte size when launching threads.

This defaults to zero, which means to use the system default.

NOTE: Windows will need to implement this.

<rdar://problem/18644448>
[UBSan] [MIPS] Adding support of UBSan for mipsel arch

Summary:
Changed files:
config-ix.cmake: Added mipsel to UBSAN_SUPPORTED_ARCH

Reviewers: rsmith, kcc, dsanders, petarj, samsonov

Reviewed By: samsonov

Subscribers: llvm-commits, mohit.bhakkad, farazs, kumarsukhani

Differential Revision: http://reviews.llvm.org/D5750