[COFF] Don't mark mingw .eh_frame sections writable

[mstorsjo]

Summary

This improves compatibility with GCC produced object files, where the .eh_frame sections are read only. With mixed flags for the involved .eh_frame sections, LLD creates two separate .eh_frame sections in the output binary, one for each flag combination, while ld.bfd probably merges them.

The previous setup of flags can be traced back to SVN r79346.

[ruiu]

LGTM

llvm/trunk/lib/MC/MCObjectFileInfo.cpp

void MCObjectFileInfo::initELFMCObjectFileInfo(const Triple &T, bool Large) {
 
   EHFrameSection =
       Ctx->getELFSection(".eh_frame", EHSectionType, EHSectionFlags);
 
   StackSizesSection = Ctx->getELFSection(".stack_sizes", ELF::SHT_PROGBITS, 0);
 }
 
 void MCObjectFileInfo::initCOFFMCObjectFileInfo(const Triple &T) {
-  EHFrameSection = Ctx->getCOFFSection(
-      ".eh_frame", COFF::IMAGE_SCN_CNT_INITIALIZED_DATA |
-                       COFF::IMAGE_SCN_MEM_READ | COFF::IMAGE_SCN_MEM_WRITE,
+  EHFrameSection =
+      Ctx->getCOFFSection(".eh_frame", COFF::IMAGE_SCN_CNT_INITIALIZED_DATA |
+                                           COFF::IMAGE_SCN_MEM_READ,
                           SectionKind::getData());
 
   // Set the `IMAGE_SCN_MEM_16BIT` flag when compiling for thumb mode.  This is
   // used to indicate to the linker that the text segment contains thumb instructions
   // and to set the ISA selection bit for calls accordingly.
   const bool IsThumb = T.getArch() == Triple::thumb;
 
   CommDirectiveSupportsAlignment = true;
 

llvm/trunk/test/MC/COFF/eh-frame.s

 // RUN: llvm-mc -filetype=obj -triple i686-pc-win32 %s -o - | llvm-readobj -s | FileCheck %s
 
   .def   _main;
   .scl  2;
   .type 32;
   .endef
   .text
   .globl  _main
 _main:
   .cfi_startproc
   ret
   .cfi_endproc
 
 // CHECK:    Name: .eh_frame
+// CHECK-NEXT:    VirtualSize:
+// CHECK-NEXT:    VirtualAddress:
+// CHECK-NEXT:    RawDataSize:
+// CHECK-NEXT:    PointerToRawData:
+// CHECK-NEXT:    PointerToRelocations:
+// CHECK-NEXT:    PointerToLineNumbers:
+// CHECK-NEXT:    RelocationCount:
+// CHECK-NEXT:    LineNumberCount:
+// CHECK-NEXT:    Characteristics [
+// CHECK-NEXT:      IMAGE_SCN_ALIGN_4BYTES
+// CHECK-NEXT:      IMAGE_SCN_CNT_INITIALIZED_DATA
+// CHECK-NEXT:      IMAGE_SCN_MEM_READ
+// CHECK-NEXT:    ]