runtime: add NaCl IRT support for 386. Native client system calls are mediated through an "outer sandbox." System calls are invoked through a stub stored in a system call table. They execute in the same address space as the caller, using the caller's stack. However, the system call interface is not part of the NaCl public API, and it is subject to change. The NaCl IRT (for "integrated runtime") is the official, stable API. In fact, the IRT is really just a thin wrapper around the system calls. It changes the function signatures somewhat, but in general there is a 1-1 correspondence with raw syscalls. This change uses the IRT for the runtime. However, we continue to support raw system calls, mainly to support the Go playground. If the IRT is available, we use the IRT function; otherwise, we invoke the raw system call. Since syscalls execute on the caller's stack, we switch to the main m->g0 stack, invoke the system call, then switch back to the original stack. Here's the skeleton of a runtime call. switch to main thread stack if !irt_is_enabled invoke raw syscall else invoke IRT function switch back to original stack The IRT is passed using the Elf aux vector. If it exists, we fetch the IRT function tables, and enable the IRT. This change also sets the linker to decouple the elf address from the file offset. The IRT data segments need to start at 0x10000000 to make room for the IRT at 0xfa00000, resulting in huge executable files if the address and file offset are the same.