Fix ADDR(nosuchsection) crash

[Nathan Sidwell]

A linker script that uses ADDR(nosuchsection) can cause confusing errors and a 
crash.  The linker script in this patch for instance, produces:

nathan@kosh:116>m68k-elf/ld/ld-new  -T 1150.ld 1150.o
m68k-elf/ld/ld-new: warning: cannot find entry symbol RAM; defaulting to 00000000
1150.o: In function `foo':
1150.c:(.text+0x6): undefined reference to `RAM'
m68k-elf/ld/ld-new: BFD 2.17.50 20061218 internal error, aborting at 
../../src/bfd/elflink.c line 6991 in elf_link_output_extsym

m68k-elf/ld/ld-new: Please report this bug.

which is less than enlightening :)  This patch changes fold-name to give an 
error when ADDR, LOADADDR or SIZEOF are given a non-existant section name.  Am I 
correct in thinking that lang_output_section_find only returns non-null when the 
section is not mentioned in the output script (it doesn't return NULL when the 
section is not output).  with this patch we get:

expr1.t:13: undefined section `ram' referenced in expression

tested on m68k-elf, ok?

nathan

-- 
Nathan Sidwell    ::   http://www.codesourcery.com   ::         CodeSourcery
nathan@codesourcery.com    ::     http://www.planetfall.pwp.blueyonder.co.uk

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