Fix a regression for devel/amd64-gcc introduced in r488642.

Restore the value of TARGETARCH to be the FreeBSD architecture
name. This fixes the block to remove float.h in the amd64
compiler. This showed up as a packing list failure on amd64-gcc,
but it also undid r475290 for amd64-gcc.

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D19126