Makefile: Set USE_PIC on x86-64

Makefile: Set USE_PIC on x86-64

On some platforms, Git.xs refuses to link with the rest of git
unless the latter is compiled with -fPIC, and we have USE_PIC
control in the Makefile for the user to set it.  At least we
know x86-64 is such, so set it in the Makefile.

The original suggestion by Marco Roeland conservatively did this
only for Linux x86-64, but let's keep the Makefile simple and if
it breaks somebody let them holler.

Signed-off-by: Junio C Hamano <junkio@cox.net>