[clang] allow -fstack-clash-protection on FreeBSD

[emaste]

Summary

-fstack-clash-protection was added in Clang commit e67cbac81211 but was enabled only on Linux. Allow it on FreeBSD as well, as it works fine.

[emaste]

We make this change in FreeBSD's in-tree Clang about 9 months ago (https://reviews.freebsd.org/D27366)

[serge-sans-paille]

You know your platform better than I do, and you already ran the patch without any issue, so LGTM

clang/lib/Driver/ToolChains/Clang.cpp

if (Arg *A = Args.getLastArg(options::OPT_mstack_protector_guard_reg_EQ)) {
     A->render(Args, CmdArgs);
   }
 }
 
 static void RenderSCPOptions(const ToolChain &TC, const ArgList &Args,
                              ArgStringList &CmdArgs) {
   const llvm::Triple &EffectiveTriple = TC.getEffectiveTriple();
 
-  if (!EffectiveTriple.isOSLinux())
+  if (!EffectiveTriple.isOSFreeBSD() && !EffectiveTriple.isOSLinux())
     return;
 
   if (!EffectiveTriple.isX86() && !EffectiveTriple.isSystemZ() &&
       !EffectiveTriple.isPPC64())
     return;
 
   if (Args.hasFlag(options::OPT_fstack_clash_protection,
                    options::OPT_fno_stack_clash_protection, false))

clang/test/Driver/stack-clash-protection.c

 // RUN: %clang -target i386-unknown-linux -fstack-clash-protection -### %s 2>&1 | FileCheck %s -check-prefix=SCP-i386
 // RUN: %clang -target i386-unknown-linux -fno-stack-clash-protection -fstack-clash-protection -### %s 2>&1 | FileCheck %s -check-prefix=SCP-i386
 // RUN: %clang -target i386-unknown-linux -fstack-clash-protection -fno-stack-clash-protection -### %s 2>&1 | FileCheck %s -check-prefix=SCP-i386-NO
 // SCP-i386: "-fstack-clash-protection"
 // SCP-i386-NO-NOT: "-fstack-clash-protection"
 
 // RUN: %clang -target x86_64-scei-linux -fstack-clash-protection -### %s 2>&1 | FileCheck %s -check-prefix=SCP-x86
+// RUN: %clang -target x86_64-unknown-freebsd -fstack-clash-protection -### %s 2>&1 | FileCheck %s -check-prefix=SCP-x86
 // SCP-x86: "-fstack-clash-protection"
 
 // RUN: %clang -target armv7k-apple-linux -fstack-clash-protection -### %s 2>&1 | FileCheck %s -check-prefix=SCP-armv7
 // SCP-armv7-NOT: "-fstack-clash-protection"
 // SCP-armv7: argument unused during compilation: '-fstack-clash-protection'
 
 // RUN: %clang -target x86_64-unknown-linux -fstack-clash-protection -S -emit-llvm -o %t.ll %s 2>&1 | FileCheck %s -check-prefix=SCP-warn
 // SCP-warn: warning: Unable to protect inline asm that clobbers stack pointer against stack clash