[arm64] Negative immediates and lack of default Neon support

[Torbjörn Granlund]

There are more problems with clang's assembler:

tmp-lshift.s:67:8: error: too few operands for instruction
.Lbx1:  ldr     x4, [x1,#-8]
        ^
tmp-lshift.s:74:2: error: too few operands for instruction
        str     x18, [x16,#-8]
        ^
tmp-lshift.s:103:2: error: too few operands for instruction
        str     x10, [x16,#-8]
        ^
tmp-lshift.s:114:2: error: too few operands for instruction
        str     x10, [x16,#-8]
        ^
tmp-lshift.s:143:2: error: too few operands for instruction
        str     x18, [x16,#-56]
        ^

The error messages are misleading, the real problem is that negative
offsets are not handled.

You Marc reported successful compilation with clang.  Did you find a
workaround for this bug?  Perhaps we really have no choice but to give
up on clang 3.4?  Now clang 3.5 is out.

-- 
Torbjörn
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