bpo-35389: platform.platform() calls libc_ver() without executable (GH-14418)

When libc_ver() is called with an executable, the
os.confstr('CS_GNU_LIBC_VERSION') fast-path cannot be taken. Modify
platform.platform() to call libc_ver() without executable, instead of
calling libc_ver(sys.executable), since sys.executable is already the
default value.