fix regression in SH/FDPIC dynamic linker

fix regression in SH/FDPIC dynamic linker

the dynamic linker was found to hang when used as the PT_INTERP, but
not when invoked as a command. the mechanism of this failure was not
determined, but the cause is clear:

commit 5552ce52000855906a5cb4f08f2e456573cca51f removed the SHARED
macro, but arch/sh/crt_arch.h is still using it to choose the right
form of the crt/ldso entry point code. moving the forced definition
from rcrt1.c to dlstart.c restores the old behavior. eventually the
logic should be changed to fully remove the SHARED macro or at least
rename it to something more reasonable.