[AArch64] Binutils/gas/ld port for ARM's new 64-bit architecture, AArch64 [4/6] gas changes

[Joseph S. Myers]

See <http://sourceware.org/ml/binutils/2010-11/msg00397.html> for details 
of the preferred way to document target assembler command-line options.  
You may not have many options now but using the approach that avoids 
duplicated text is still a good idea.

-- 
Joseph S. Myers
joseph@codesourcery.com