MFC r345676:

powerpc64: Use medium code model in asm files for TOC references

Summary:
With a sufficiently large TOC, it's possible to index out of range, as
the immediate load instructions only permit 16-bit indices, allowing up
to 64kB range (signed) from the base pointer. Allow +/- 2GB range, with
the medium code model TOC accesses in asm.

Patch originally by Brandon Bergren. The issue appears to impact ELFv2
more than ELFv1.

Reviewed by: luporl
Differential Revision: https://reviews.freebsd.org/D19708