[gold-plugin] allow function/data sections to be toggleable

[nickdesaulniers]

Summary

r336838 allowed these to be toggleable.
r336858 reverted r336838.
r336943 made the generation of these sections conditional on LDPO_REL.

This commit brings back the toggle-ability. You can specify:
-plugin-opt=-function-sections
-plugin-opt=-data-sections
For your linker flags to disable the changes made in r336943.

Without toggling r336943 off, arm64 linux kernels linked with gold-plugin
see significant boot time regressions, but with r336943 outright reverted
x86_64 linux kernels linked with gold-plugin fail to boot.

[pcc]

The commit message should explain why we are making these toggleable.

[pcc]

LGTM with s/r336943/r336838/g and s/r336838/r336943/g in commit message.

llvm/trunk/tools/gold/gold-plugin.cpp

static std::unique_ptr<LTO> createLTO(IndexWriteCallback OnIndexWrite,
   Conf.CPU = options::mcpu;
   Conf.Options = InitTargetOptionsFromCodeGenFlags();
 
   // Disable the new X86 relax relocations since gold might not support them.
   // FIXME: Check the gold version or add a new option to enable them.
   Conf.Options.RelaxELFRelocations = false;
 
   // Toggle function/data sections.
+  if (FunctionSections.getNumOccurrences() == 0)
     Conf.Options.FunctionSections = SplitSections;
+  if (DataSections.getNumOccurrences() == 0)
     Conf.Options.DataSections = SplitSections;
 
   Conf.MAttrs = MAttrs;
   Conf.RelocModel = RelocationModel;
   Conf.CodeModel = getCodeModel();
   Conf.CGOptLevel = getCGOptLevel();
   Conf.DisableVerify = options::DisableVerify;
   Conf.OptLevel = options::OptLevel;
   if (options::Parallelism)