[RFC][AArch64] Support in-tree building with libgloss and linker scripts (specs files)

[Richard Earnshaw <rearnsha at arm dot com>]

When using libgloss and linker scripts unified source tree builds
(gcc+newlib+binutils) fail when building the libraries because the
linker scripts have not been installed at that time.

A suitable solution to this is to make copies of the scripts in the
build area, which the compiler is configured to search during the build.
 Then using LDFLAGS_FOR_TARGET=-specs=<spec-file> flag will correctly
pick up the appropriate libgloss specs file.

I've marked this as an RFC as I'm not sure if the pattern-based rule is
GNUmake specific, and if it is, whether this is not appropriate for
newlib -- it could be written out long-hand, but this rule makes the
construct trivial to port to other libgloss targets.

This was the only patch I needed to make an aarch64-elf unified tree
build and run the gcc testsuite.

Thoughts please?

R.

	* aarch64/Makefile.in (RDIMON_SCRIPTS): Build local copies
	during make all.

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