fix bug for add/sub with r8-r15

I had been setting the REX R bit to select high registers,
but you actually set the B bit.  Don't know how I got that
wrong before... the leading byte should be 49 not 4c.

    $ cat test.s
    foo:
        addq $7, %r8

    $ clang -c test.s && objdump -d test.o

    0000000000000000 <foo>:
       0:	49 83 c0 07          	add    $0x7,%r8

Change-Id: I039e1c4f4ea20523a1e2cc9bcf5f6d9321a6223b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/227177
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
2 files changed