modpost: clean up is_executable_section()

SHF_EXECINSTR is a bit flag (#define SHF_EXECINSTR 0x4).
Compare the masked flag to '!= 0'.

There is no good reason to stop modpost immediately even if a special
section index is given. You will get a section mismatch error anyway.

Also, change the return type to bool.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>