[MC][ELF] Introduce .rodata.cold section

[Amir]

Summary

Introduce .rodata.cold section that is used for infrequently accessed
data to improve cache utilization.

Background: BOLT performs an optimization of moving unused jump tables
to .rodata.cold section. More details are here:
BOLT

[MaskRay]

The default section variables are there because they are referenced in many places internally.
I don't see that .rodata.cold needs to be a built-in default section. The application can call Ctx->getELFSection(".rodata.cold", ELF::SHT_PROGBITS, ELF::SHF_ALLOC); instead.

[Amir]

@MaskRay: makes sense. Will just use MCObjectFileInfo->getContext()... interface since we're only using in a single place.

llvm/include/llvm/MC/MCObjectFileInfo.h

protected:
   MCSection *BSSSection = nullptr;
 
   /// Section that is readonly and can contain arbitrary initialized data.
   /// Targets are not required to have a readonly section. If they don't,
   /// various bits of code will fall back to using the data section for
   /// constants.
   MCSection *ReadOnlySection = nullptr;
 
+  /// Same as the above but for infrequently used data.
+  MCSection *ReadOnlyColdSection = nullptr;
+
   /// If exception handling is supported by the target, this is the section the
   /// Language Specific Data Area information is emitted to.
   MCSection *LSDASection = nullptr;
 
   /// If exception handling is supported by the target and the target can
   /// support a compact representation of the CIE and FDE, this is the section
   /// to emit them into.
   MCSection *CompactUnwindSection = nullptr;
public:
   unsigned getCompactUnwindDwarfEHFrameOnly() const {
     return CompactUnwindDwarfEHFrameOnly;
   }
 
   MCSection *getTextSection() const { return TextSection; }
   MCSection *getDataSection() const { return DataSection; }
   MCSection *getBSSSection() const { return BSSSection; }
   MCSection *getReadOnlySection() const { return ReadOnlySection; }
+  MCSection *getReadOnlyColdSection() const { return ReadOnlyColdSection; }
   MCSection *getCompactUnwindSection() const { return CompactUnwindSection; }
   MCSection *getDwarfAbbrevSection() const { return DwarfAbbrevSection; }
   MCSection *getDwarfInfoSection() const { return DwarfInfoSection; }
   MCSection *getDwarfInfoSection(uint64_t Hash) const {
     return getDwarfComdatSection(".debug_info", Hash);
   }
   MCSection *getDwarfLineSection() const { return DwarfLineSection; }
   MCSection *getDwarfLineStrSection() const { return DwarfLineStrSection; }

llvm/lib/MC/MCObjectFileInfo.cpp

TextSection = Ctx->getELFSection(".text", ELF::SHT_PROGBITS,
                                    ELF::SHF_EXECINSTR | ELF::SHF_ALLOC);
 
   DataSection = Ctx->getELFSection(".data", ELF::SHT_PROGBITS,
                                    ELF::SHF_WRITE | ELF::SHF_ALLOC);
 
   ReadOnlySection =
       Ctx->getELFSection(".rodata", ELF::SHT_PROGBITS, ELF::SHF_ALLOC);
 
+  ReadOnlyColdSection =
+      Ctx->getELFSection(".rodata.cold", ELF::SHT_PROGBITS, ELF::SHF_ALLOC);
+
   TLSDataSection =
       Ctx->getELFSection(".tdata", ELF::SHT_PROGBITS,
                          ELF::SHF_ALLOC | ELF::SHF_TLS | ELF::SHF_WRITE);
 
   TLSBSSSection = Ctx->getELFSection(
       ".tbss", ELF::SHT_NOBITS, ELF::SHF_ALLOC | ELF::SHF_TLS | ELF::SHF_WRITE);
 
   DataRelROSection = Ctx->getELFSection(".data.rel.ro", ELF::SHT_PROGBITS,