MIPS: Fix exception handler memcpy()

The exception handler subroutines are declared as a single char, but
when copied to the required addresses the copy length is 0x80.

When range checks are enabled for memcpy() this results in a build
failure, with error messages such as:

In file included from arch/mips/mti-malta/malta-init.c:15:
In function 'memcpy',
    inlined from 'mips_nmi_setup' at arch/mips/mti-malta/malta-init.c:98:2:
include/linux/string.h:376:4: error: call to '__read_overflow2' declared with attribute error: detected read beyond size of object passed as 2nd parameter
  376 |    __read_overflow2();
      |    ^~~~~~~~~~~~~~~~~~

Change the declarations to use type char[].

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: YunQiang Su <syq@debian.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>