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

[mcgrathr]

Summary

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

[phosek]

LGTM

[mcgrathr]

Can someone land this for me?

lib/Target/X86/X86ISelLowering.cpp

 
 Value *X86TargetLowering::getIRStackGuard(IRBuilder<> &IRB) const {
   // glibc, bionic, and Fuchsia have a special slot for the stack guard in
   // tcbhead_t; use it instead of the usual global variable (see
   // sysdeps/{i386,x86_64}/nptl/tls.h)
   if (hasStackGuardSlotTLS(Subtarget.getTargetTriple())) {
     if (Subtarget.isTargetFuchsia()) {
       // <magenta/tls.h> defines MX_TLS_STACK_GUARD_OFFSET with this value.
-      return SegmentOffset(IRB, 0x10, 257);
+      return SegmentOffset(IRB, 0x10, getAddressSpace());
     } else {
       // %fs:0x28, unless we're using a Kernel code model, in which case
       // it's %gs:0x28.  gs:0x14 on i386.
       unsigned Offset = (Subtarget.is64Bit()) ? 0x28 : 0x14;
       return SegmentOffset(IRB, Offset, getAddressSpace());
     }
   }
 
if (Subtarget.isTargetAndroid()) {
     // %gs:0x24 on i386
     unsigned Offset = (Subtarget.is64Bit()) ? 0x48 : 0x24;
     return SegmentOffset(IRB, Offset, getAddressSpace());
   }
 
   // Fuchsia is similar.
   if (Subtarget.isTargetFuchsia()) {
     // <magenta/tls.h> defines MX_TLS_UNSAFE_SP_OFFSET with this value.
-    return SegmentOffset(IRB, 0x18, 257);
+    return SegmentOffset(IRB, 0x18, getAddressSpace());
   }
 
   return TargetLowering::getSafeStackPointerLocation(IRB);
 }
 
 bool X86TargetLowering::isNoopAddrSpaceCast(unsigned SrcAS,
                                             unsigned DestAS) const {
   assert(SrcAS != DestAS && "Expected different address spaces!");