${MAKE} bootstrap: Build bootkits with -static-libgcc

We don't want our bootkits to have a run-time dependency on libgcc. In
fact GHC's implementation of Haskell exception handling does not
depend on libgcc's facilities so it is attractive to do the same for
"normal" build... but we can't. This is because Haskell programs may
call C functions via FFI, and those C functions may call C++ functions
in turn, possibly in a different shared library.

But on some platforms, gcc automagically inserts a dependency on a
shared libgcc when -lpthread is given, which is seemingly unavoidable.

(pho)