clang: Enable unwind tables on !amd64

There doesn't seem to be much sense in defaulting "on" unwind tables on
amd64 and not on other arches.  It causes surprising differences between
platforms, such as the PR below.

Prior to this change, FreeBSD inherited the default implementation of the
method from the Gnu.h Generic_Elf => Generic_GCC parent class, which
returned true only for amd64 targets.  Override that and opt on always,
similar to, e.g., NetBSD.

PR:		241562
Reported by:	lwhsu
Reviewed by:	dim
Discussed with:	emaste
MFC after:	I'm not going to, but you should feel free
Relnotes:	yes
Differential Revision:	https://reviews.freebsd.org/D22252

contrib/llvm/tools/clang/lib/Driver/ToolChains/FreeBSD.cpp

@@ -409,6 +409,8 @@ llvm::ExceptionHandling FreeBSD::GetExceptionModel(const ArgList &Args) const {
 
 bool FreeBSD::HasNativeLLVMSupport() const { return true; }
 
+bool FreeBSD::IsUnwindTablesDefault(const ArgList &Args) const { return true; }
+
 bool FreeBSD::isPIEDefault() const { return getSanitizerArgs().requiresPIE(); }
 
 SanitizerMask FreeBSD::getSupportedSanitizers() const {

contrib/llvm/tools/clang/lib/Driver/ToolChains/FreeBSD.h

@@ -67,6 +67,7 @@ public:
 
   llvm::ExceptionHandling GetExceptionModel(
       const llvm::opt::ArgList &Args) const override;
+  bool IsUnwindTablesDefault(const llvm::opt::ArgList &Args) const override;
   bool isPIEDefault() const override;
   SanitizerMask getSupportedSanitizers() const override;
   unsigned GetDefaultDwarfVersion() const override;