[Driver] Disable uwtable by default in -ffreestanding mode

[Driver] Disable uwtable by default in -ffreestanding mode

We make the same decision when compiling the kernel or kexts -- we
should do this in -ffreestanding mode as well to avoid size regressions
in a potentially large set of firmware projects.

It's still possible to get uwtable information in -ffreestanding mode by
compiling with -funwind-tables (I expect this to be a rare case: I
certainly haven't seen any projects like that).

Context: -munwind-tables was enabled by default for some arm targets in
r310006.

Testing: check-clang

rdar://problem/33934446

Differential Revision: https://reviews.llvm.org/D37777

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@313087 91177308-0d34-0410-b5e6-96231b3b80d8