Speed up symbol lookup for the amd64 kernel modules.

Amd64 uses relocatable object files as the modules format. It is good
WRT not having unneeded overhead for PIC code, in particular, due to
absence of useless GOT and PLT. But the cost is that the module
linking process cannot use hash to speed up the symbol lookup, and
that each reference to the symbol requiring a relocation, instead of
single-place relocation in GOT.

Cache the successfull symbol lookup results in the module symbol
table, using the newly allocated SHN_FBSD_CACHED value from
SHN_LOOS-HIOS range as an indicator. The SHN_FBSD_CACHED together
with the non-existent definition of the found symbol are reverted
after successfull relocations, which is done under kld_sx lock, so it
should not be visible to other consumers of the symbol table.

Submitted by: Conrad Meyer
Differential Revision: https://reviews.freebsd.org/D1718
MFC after: 3 weeks