Key test case: nixpkgs#pkgsStatic.pkgsLLVM.ncurses
Prior to this patch, this fails with errors such as:
```
error: undefined symbol: __cxa_throw
```
I think this is a reasonable solution because in #292043, libcxxabi was
'merged into libcxx', however, the commit message suggests that only
dynamic linking was accounted for, because it says:
```
* linux/freebsd `libc++.so` is a linker script `LINK(libc++.so.1, -lc++abi)` making `-lc++` sufficient.
```
Whereas, I found that if I tried linking a "hello world" C++ program
with a static hostPlatform, it failed unless -lc++abi was passed.
Signed-off-by: Peter Waller <p@pwaller.net>
3c03811683