[ARM] Dynamic stack alignment for 16-bit Thumb

[ARM] Dynamic stack alignment for 16-bit Thumb

This patch implements dynamic stack (re-)alignment for 16-bit Thumb. When
targeting processors, which support only the 16-bit Thumb instruction set
the compiler ignores the alignment attributes of automatic variables and may
silently generate incorrect code.

Differential revision: https://reviews.llvm.org/D38143

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316289 91177308-0d34-0410-b5e6-96231b3b80d8