[Fuchsia] Use %gs for ABI slots under -mcmodel=kernel

Make x86_64-fuchsia targets under -mcmodel=kernel use %gs rather
than %fs to access ABI slots for stack-protector and safe-stack

Patch by Roland McGrath.

Differential Revision: https://reviews.llvm.org/D30870

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298302 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed