Merge branch 'Add clang-based BTF_KIND_FLOAT tests'

Ilya Leoshkevich says:

====================

The two tests here did not make it into the main BTF_KIND_FLOAT series
because of dependency on LLVM.

v0: https://lore.kernel.org/bpf/20210226202256.116518-10-iii@linux.ibm.com/
v1 -> v0: Per Alexei's suggestion, document the required LLVM commit.

v1: https://lore.kernel.org/bpf/20210305170844.151594-1-iii@linux.ibm.com/
v1 -> v2: Per Andrii's suggestions, use double in
          core_reloc_size___diff_sz and non-pointer member in
          btf_dump_test_case_syntax.
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>