[MC][ELF] Don't emit .note.GNU-stack sections on Solaris

LLVM currently emits `.note.GNU-stack` sections on all ELF targets.

However, Solaris ld doesn't know/care about them.  Even worse, with the
revised Solaris GNU ld patch (D85309 <https://reviews.llvm.org/D85309>),
there are hundreds of warnings:

  /usr/gnu/bin/ld: warning: /usr/lib/amd64/crtn.o: missing .note.GNU-stack
section implies executable stack
  /usr/gnu/bin/ld: NOTE: This behaviour is deprecated and will be removed
in a future version of the linker

The Solaris crts are not going to change here, and even if they were, GNU
ld would emit `PT_GNU_STACK` segments that Solaris `ld.so.1` ignores.

So the note sections are completely useless on Solaris and this patch
disables their creation.

Instead, Solaris has its own mechanisms to control stack executability:
`PT_SUNW_STACK`, `DT_SUNW_SX_NXSTACK` and the system-wide control via
`sxadm` where `nxstack` defaults to on.

Tested on `amd64-pc-solaris2.11` and `sparcv9-sun-solaris2.11` with Solaris
ld and GNU ld, and `x86_64-pc-linux-gnu`.

Differential Revision: https://reviews.llvm.org/D159179

llvm/lib/MC/MCAsmInfoELF.cpp

@@ -21,6 +21,10 @@ using namespace llvm;
 void MCAsmInfoELF::anchor() {}
 
 MCSection *MCAsmInfoELF::getNonexecutableStackSection(MCContext &Ctx) const {
+  // Solaris doesn't know/doesn't care about .note.GNU-stack sections, so
+  // don't emit them.
+  if (Ctx.getTargetTriple().isOSSolaris())
+    return nullptr;
   return Ctx.getELFSection(".note.GNU-stack", ELF::SHT_PROGBITS, 0);
 }
 

llvm/test/CodeGen/X86/execstack.ll

@@ -0,0 +1,9 @@
+;; Check that .note.GNU-stack sections are emitted on Linux, but not on Solaris.
+
+; RUN: llc < %s -mtriple=i686-linux | FileCheck %s -check-prefix=CHECK-GNUSTACK
+; RUN: llc < %s -mtriple=x86_64-linux | FileCheck %s -check-prefix=CHECK-GNUSTACK
+; RUN: llc < %s -mtriple=i386-solaris | FileCheck %s -check-prefix=CHECK-NOGNUSTACK
+; RUN: llc < %s -mtriple=amd64-solaris | FileCheck %s -check-prefix=CHECK-NOGNUSTACK
+
+; CHECK-GNUSTACK: .section	".note.GNU-stack","",@progbits
+; CHECK-NOGNUSTACK-NOT: .section	".note.GNU-stack","",@progbits