kbuild: lto: Merge module sections if and only if CONFIG_LTO_CLANG is enabled

Merge module sections only when using Clang LTO. With ld.bfd, merging
sections does not appear to update the symbol tables for the module,
e.g. 'readelf -s' shows the value that a symbol would have had, if
sections were not merged. ld.lld does not show this problem.

The stale symbol table breaks gdb's function disassembler, and presumably
other things, e.g.

  gdb -batch -ex "file arch/x86/kvm/kvm.ko" -ex "disassemble kvm_init"

reads the wrong bytes and dumps garbage.

Fixes: dd277622

 ("kbuild: lto: merge module sections")
Cc: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210322234438.502582-1-seanjc@google.com