objtool: Fix __mcount_loc generation with Clang's assembler

When objtool generates relocations for the __mcount_loc section, it
tries to reference __fentry__ calls by their section symbol offset.
However, this fails with Clang's integrated assembler as it may not
generate section symbols for every section. This patch looks up a
function symbol instead if the section symbol is missing, similarly
to commit e81e0724

 ("objtool: Support Clang non-section symbols
in ORC generation").

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>