fix AArch64 predicate-as-counter register printed in hex

[yufengzjj]

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

AArch64 predicate-as-counter registers pn10–pn15 were printed in hexadecimal
(e.g. ptrue pn0xa.h instead of ptrue pn10.h).

The cause is in the printPredicateAsCounter macro in arch/AArch64/AArch64InstPrinter.c.
It printed the register index via printUInt32(), which automatically switches to a
0x%x hex representation once the value exceeds HEX_THRESHOLD (defined as 9 in utils.h):

void printUInt32(SStream *ss, uint32_t val) {
    if (val > HEX_THRESHOLD)
        SStream_concat(ss, "0x%x", val);   // pn10 -> "pn0xa"
    else
        SStream_concat(ss, "%u", val);
}

So pn0–pn9 printed correctly, but the index 10–15 (registers pn10–pn15)
was rendered as 0xa–0xf.

The register number must always be printed in decimal, matching upstream LLVM
(O << "pn" << (Reg - AArch64::PN0)). The fix prints the index directly in decimal
instead of routing it through the hex-thresholding helper:

SStream_concat(O, "%s", "pn");

- printUInt32(O, (Reg - AArch64_PN0));
+ SStream_concat(O, "%u", (Reg - AArch64_PN0));

This is a one-line, output-only change; the decoded register value/detail is unaffected.

Test plan

Added a regression test case to tests/issues/issues.yaml for ptrue pn10.h
(bytes [0x12, 0x78, 0x60, 0x25]).

Important

the test runners (cstest / cstest_py) normalize hex numbers to decimal when
comparing asm_text (see replace_hex() in suite/cstest/src/helper.c and
normalize_asm_text() in cstest_py), so an asm_text-only check passes even with the
bug present(e.g. tests\MC\AArch64\SVE2p1\ptrue.s.yaml).
To make the test actually catch the regression, the expected op_str field
is set explicitly (op_str: "pn10.h").

Verified locally with cstest/cstest_py
Also confirmed via cstool directly

Closing issues
#2973

[Rot127]

the test runners (cstest / cstest_py) normalize hex numbers to decimal when

Yes, this is one of the shortcomings of testing I added because I didn't want to fix all the old tests I just moved.