kernel should not reject -mips2 ELF binaries

["Jay Carlson" <nop@nop.com>]

include/asm-mips/elf.h contains a macro, elf_check_arch, that decides if an
executable is plausible to run under this kernel.  It currently accepts
binaries flagged as MIPS1 ISA, and rejects all other ISAs.

#define elf_check_arch(hdr)                                             \
({                                                                      \
        int __res = 1;                                                  \
        struct elfhdr *__h = (hdr);                                     \
                                                                        \
        if ((__h->e_machine != EM_MIPS) &&                              \
            (__h->e_machine != EM_MIPS_RS4_BE))                         \
                __res = 0;                                              \
        if (__h->e_flags & EF_MIPS_ARCH)                                \
                __res = 0;                                              \
                                                                        \
        __res;                                                          \
})

I think we should make an exception for MIPS2.  Turns out that the two Linux
VR processor families benefit from some of the MIPS II features; most
notably, code density is improved by eliminating load delay slots.  If I
build executables that take advantage of this, they legitimately should be
flagged with E_MIPS_ARCH_2 (since they won't run on my decstation).

So what's the right way to fix this?  Three things come to mind:

1) rip out the EF_MIPS_ARCH check from elf_check_arch.
2) compare the value with E_MIPS_ARCH_1 and E_MIPS_ARCH_2
3) figure out what the capabilities of the current processor are and reject
E_MIPS_ARCH2 on R2/3000s.

I'd just go do #3 except it looks like a bigger pain than it's worth.  No
other linux kernel port goes to that amount of trouble, of course.

(I may do #1 or #2 in the Linux VR CVS as a stopgap.)

Jay