libcc1 has a memory leak when calling fork_exec -- it allocates a new
vector of arguments, but then does not free it anywhere.  This patch
changes this code to use std::vector instead.

Note that the previous code tried to avoid bad_alloc.  I don't believe
this is very important.  For one thing, plenty of other allocations do
not bother with this.

libcc1

	* gdbctx.hh (do_compile): Use std::vector.