PR24408 @{u,k}register(regno) index/context checking

PR24408 @{u,k}register(regno) index/context checking

wcohen discovered that the (guru-mode) @*register operator doesn't
sufficiently check the context it is run in, possibly derefencing
null context->*regs pointers, or going out-of-bounds with register
numbering.  This code adds checking via a generic runtime/**/loc2c*
check_register_{fetch,store} macro.  It is used as a wrapper for
all architectures for both kernel and user register
fetch/store ops.