[BPF] use std::map to ensure consistent output

The .BTF.ext FuncInfoTable and LineInfoTable contain
information organized per ELF section. Current definition
of FuncInfoTable/LineInfoTable is:
  std::unordered_map<uint32_t, std::vector<BTFFuncInfo>> FuncInfoTable
  std::unordered_map<uint32_t, std::vector<BTFLineInfo>> LineInfoTable
where the key is the section name off in the string table.
The unordered_map may cause the order of section output
different for different platforms.

The same for unordered map definition of
  std::unordered_map<std::string, std::unique_ptr<BTFKindDataSec>>
    DataSecEntries
where BTF_KIND_DATASEC entries may have different ordering
for different platforms.

This patch fixed the issue by using std::map.
Test static-var-derived-type.ll is modified to generate two
DataSec's which will ensure the ordering is the same for all
supported platforms.

Signed-off-by: Yonghong Song <yhs@fb.com>
llvm-svn: 357077