Fix #122: gcc 11 can't build cmucl
The main problem is from defining _GNU_SOURCE on Linux for all files. To fix this:
- Remove
-D_GNU_SOURCE
from the makefiles for linux (both gcc and clang). - Add a new reg_EFL constant to be used by SC_EFLAGS in x86-lispregs.h
- Define _GNU_SOURCE in Linux-os.c so that we can get REG_foo values needed to access the appropriate entry in uc_mcontext.gregs[].