[VxWorks] Commonise general OS-specific macros

[Nathan Sidwell]

Richard Sandiford wrote:
> This patch defines a new VXWORKS_OS_CPP_BUILTINS macro that can be
> used by all VxWorks targets.  It also moves some defines from CPP_SPEC
> to this new macro.
> 
> The patch only changes the x86 port to use VXWORKS_OS_CPP_BUILTINS;
> we'll convert other targets later.
> 
> Tested on i586-wrs-vxworks.  OK to install?

ok thanks.

nathan
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