Build libc-start with stack protector for SHARED

This does not change the emitted code since __libc_start_main does not
return, but is important for formal flags compliance.

This also cleans up the cosmetic inconsistency in the stack protector
flags in csu, especially the incorrect value of STACK_PROTECTOR_LEVEL.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>