[PATCH][GAS] aarch64: Add DSB instruction Armv8.7-a variant

[Joseph Myers]

This breaks building GCC (master) for aarch64-linux-gnu.  Building libgcc, 
I see:

/tmp/ccYijJ81.s:79: Error: missing immediate expression at operand 1 -- `dsb ish'
/tmp/ccYijJ81.s:124: Error: missing immediate expression at operand 1 -- `dsb ish'
/scratch/jmyers/glibc/many11/src/gcc/libgcc/static-object.mk:17: recipe for target 'sync-cache.o' failed

If that error is intended, there needs to be a corresponding GCC patch to 
fix the build with new binutils.

-- 
Joseph S. Myers
joseph@codesourcery.com