tune-corei7.inc: update TUNE_CCARGS -march CPU type corei7 -> nehalem

tune-corei7.inc: update TUNE_CCARGS -march CPU type corei7 -> nehalem

The gcc "corei7" CPU type was last documented in gcc 4.8.x and has
been undocumented from gcc 4.9.x onwards:

  https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/i386-and-x86-64-Options.html
  https://gcc.gnu.org/onlinedocs/gcc-4.9.4/gcc/i386-and-x86-64-Options.html

Although it still seems to be accepted by gcc 7.x, it's likely to be
deprecated and removed at some point. To preempt that, switch the
corei7 TUNE_CCARGS -march CPU type to "nehalem", which is the closest
replacement (and matches the CPU type already being passed to qemu).

Since the tune-corei7.inc include file is intended to cover a range
of CPUs from Nehalem onwards, switch the TUNE_CCARGS -mtune option
from "corei7" to "generic", which instructs gcc to produce code
optimized for the most common IA32/AMD64/EM64T processors.

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>