arch/x86: (Intel64) optimize and re-order startup assembly sequence

In some places the code was being overly pedantic; e.g., there is no
need to load our own 32-bit descriptors because the loader's are fine
for our purposes. We can defer loading our own segments until 64-bit.

The sequence is re-ordered to faciliate code sharing between the BSP
and APs when SMP is enabled (all BSP-specific operations occur before
the per-CPU initialization).

Signed-off-by: Charles E. Youse <charles.youse@intel.com>