chacha/asm/chacha-x86.pl: improve [backward] portability.

In order to minimize dependency on assembler version a number of
post-SSE2 instructions are encoded manually. But in order to simplify
the procedure only register operands are considered. Non-register
operands are passed down to assembler. Module in question uses pshufb
with memory operands, and old [GNU] assembler can't handle it.
Fortunately in this case it's possible skip just the problematic
segment without skipping SSSE3 support altogether.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>