Unifiy the cuda toolchain definition of gcc/nvcc and cuda-clang.

gcc >= 7 will change how it treats -pie [1]; passing -pie after -shared
on the command line is not possible any more; given that the legacy way to
configure flags in the gcc/nvcc toolchain does not allow control over where
the flags go or how to provide -pie only for linking of binaries, we can
prevent this from breaking in the future by also using the new feature
mechanism for gcc/nvcc.

In addition to moving the gcc-specific workarounds in the toolchain to
cuda_configure.bzl, document them, so we don't need to rediscover them in the
future.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77464

PiperOrigin-RevId: 197522719