Add core.sys.posix.fcntl.O_* flag definitions for ARM.

Linux 3.6.10 on 32 bit ARM.

This is a back-port of D-Programming-Language/druntime#629.
Fix Fiber size calculation for ucontext_t systems.

This case is notably hit on (32 bit) ARM, where ucontext_t is
8 byte aligned.

This is a back-port of D-Programming-Language#druntime/628.
Added core.sys.posix.sys.socket declarations for ARM.

From Linux 3.10.6 on 32 bit ARM.

This is a back-port of D-Programming-Language/druntime#627.
ARM: First C vararg implementation, copied from the x86 one.

This is likely broken in subtle ways, but for now, everything
that compiles is good.
ARM: Implement EH register accessors.