Linux: Build hybrid library with Genode tool chain

Using the host compiler in this case seems to be an artifact from an
older change. On x86_64, this approach ended in unsable hybrid binaries
due to incompatible handling of non-trivial return values, i.e.
structures. See '-freg-struct-return' in GCC manual page:

  "[...] If there is no standard convention, GCC defaults to
  -fpcc-struct-return, except on targets where GCC is the principal
  compiler.  In those cases, we can choose the standard, and we chose
  the more efficient register return alternative."

In other words: All x86_64 Linux systems break the ABI standard :-(

base-linux/lib/mk/lx_hybrid.mk

@@ -3,5 +3,3 @@ LIBS  += syscall env
 
 vpath new_delete.cc $(BASE_DIR)/src/base/cxx
 vpath lx_hybrid.cc   $(REP_DIR)/src/platform
-
-CUSTOM_CXX = g++