gcc recommends -Og for debugging; follow its advice.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

make[1]: Entering directory `/home/zwu/virt/osv/build/release'
  GEN gen/include/bits/alltypes.h
  AS arch/x64/boot16.o
  LD boot.bin
  AS arch/x64/boot.o
  CXX loader.o
In file included from <command-line>:0:0:
./../../compiler/include/intrinsics.hh:10:127: error: new declaration ‘unsigned char __builtin_ia32_addcarryx_u64(unsigned char, long unsigned int, long unsigned int, long long unsigned int*)’ [-Werror]
./../../compiler/include/intrinsics.hh:10:127: error: ambiguates built-in declaration ‘unsigned char __builtin_ia32_addcarryx_u64(unsigned char, long long unsigned int, long long unsigned int, long long unsigned int*)’ [-Werror]
cc1plus: all warnings being treated as errors
make[1]: *** [loader.o] Error 1
make[1]: Leaving directory `/home/zwu/virt/osv/build/release'
make: *** [all] Error 2

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>

gcc 4.7 doesn't define a few builtins, which we don't use, but the 4.8
headers require.

Declare them so compilation passes.

__attribute__((cold)) on labels isn't supported by gcc 4.7.  Detect support
for the feature and enable it conditionally.