x86: fix access metadata for EVEX writemasks

[raaller]

Your checklist for this pull request

• I've documented or updated the documentation of every API function and struct this PR changes.
• I've added tests that prove my fix is effective or that my feature works (if possible)

Detailed description

EVEX writemasks are exposed by the x86 printers as additional "cs_x86_op" operands, while the instruction access table describes only the non-writemask semantic operands. Using the public operand position directly therefore shifts the access lookup.

With Intel syntax, operands after the writemask can lose their access metadata. With AT&T syntax, the writemask itself is appended after the semantic operands and receives no access metadata. This change records the operand created from "ENCODING_WRITEMASK" and its corresponding detail-operand position. The printers assign "CS_AC_READ" to that operand and exclude it from the semantic access index. K registers used as regular instruction operands are left unchanged.

Test plan

Added seven regression tests covering:

• Intel and AT&T syntax;
• zeroing and merging masks;
• register and memory sources;
• an unmasked control case;
• a second masked opcode ("VMOVDQU64");
• an instruction with an immediate operand ("VPSLLD").

Closing issues

Closes #2565