xlimits.h for embedded targets

[Christian Iseli]

> Is this just a procedural change with no real impact on when the file is
> built or the contents of the resulting file? 

The impact is that the proposed change lets the target file t-target overide the way
limits.h is constructed.  For example, my target file t-c816 defines it as

# Target specific header files
LIMITS_H = limits-c816.h

limits-c816.h: $(srcdir)/config/c816/limits-c816.h msc816.inc
	cp $(srcdir)/config/c816/limits-c816.h limits-c816.h

Thus the file will be "constructed" before it is needed by msc816.inc, which is a file
containing some assembler macros, which is needed when the compiler driver assembles
some cc1 output.

					Christian