sh: SMP support for SH2 entry.S

The SH2 version of entry.S uses global variables, which need to be
cpu-local in order to work with SMP. For ease of access from asm,
simply use arrays indexed by cpu number, and require the availability
of an address (mmio register or properly setup per-cpu memory) from
which the current cpu's index can be read.

Signed-off-by: Rich Felker <dalias@libc.org>