cgen: nva needs position independent code

cgen: nva needs position independent code

When building with clang on FreeBSD 14 we are getting:

[ 43%] Generating nvapy.c
[ 43%] Building C object nva/CMakeFiles/nvapy.dir/nvapy.c.o
[ 44%] Building C object nva/CMakeFiles/nvapy.dir/nva.c.o
[ 44%] Building C object nva/CMakeFiles/nvapy.dir/__/nvhw/chipset.c.o
[ 44%] Linking C shared module nvapy.so
ld: error: relocation R_X86_64_64 cannot be used against local symbol; recompile with -fPIC
>>> defined in ../cgen/libcgen.a(gpu.c.o)
>>> referenced by gpu.c
>>>               gpu.c.o:(gpu_list) in archive ../cgen/libcgen.a

This fixes this error.