[BPF] Fix .BTF.ext reloc type assigment issue

Commit f1db33c5c1a9 ("[BPF] Disable relocation for .BTF.ext section")
assigned relocation type R_BPF_NONE if the fixup type
is FK_Data_4 and the symbol is temporary.
The reason is we use FK_Data_4 as a fixup type
for insn offsets in .BTF.ext section.

Just checking whether the symbol is temporary is not enough.
For example, .debug_info may reference some strings whose
fixup is FK_Data_4 with a temporary symbol as well.

To truely reflect the case for .BTF.ext section,
this patch further checks that the section associateed with the symbol
must be SHF_ALLOC and SHF_EXECINSTR, i.e., in the text section.
This fixed the above-mentioned problem.

Signed-off-by: Yonghong Song <yhs@fb.com>
llvm-svn: 350637