[gcc] multilib mode: build both x86-32 and x86-64 libraries

in the patch:
* combined linux.h and i386/linux64.h into i386/nacl.h
* removed LINUX target specifics when it was easy
* target multilib definitions: i386/t-nacl64
* config.gcc cleanup

side changes:
* define __linux__ and Co in GCC to let Chrome headers build smoothly
* disable llrint() on x86-64 in newlib to allow some examples compile on nacl64
3 files changed