gnu: nasm: Make build bit-reproducible.

* gnu/packages/patches/nasm-no-ps-pdf.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/assembly.scm (nasm): Use it.  Remove ghostscript, do not
build PS or PDF docs.  Makes build bit-reproducible.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>