[BPF] fix flaky btf unit test static-var-derived-type.ll

The DataSecEentries is defined as an unordered_map since
order does not really matter.
  std::unordered_map<std::string, std::unique_ptr<BTFKindDataSec>>
      DataSecEntries;
This seems causing the test static-var-derived-type.ll flaky
as two sections ".bss" and ".readonly" have undeterministic
ordering when performing map iterating, which decides the
output assembly code sequence of BTF_KIND_DATASEC entries.

Fix the test to have only one data section to remove
flakiness.

Signed-off-by: Yonghong Song <yhs@fb.com>
llvm-svn: 356731