boot-qemu.sh: Use implementation defined pointer authentication algorithm

[nathanchance]

By default, QEMU's TCG uses the architected QARMA algorithm for pointer
authentication, which is better cryptographically but extremely slow to
emulate. As of QEMU 6.0.0, there is an "Implementation Defined"
algorithm available, which is not cryptographic but significantly faster
to run.

ARCH=arm64 defconfig:

Benchmark 1: QARMA
  Time (mean ± σ):     10.381 s ±  0.048 s    [User: 8.469 s, System: 0.142 s]
  Range (min … max):   10.317 s … 10.478 s    50 runs

Benchmark 2: Implementation Defined
  Time (mean ± σ):      7.051 s ±  0.015 s    [User: 5.125 s, System: 0.130 s]
  Range (min … max):    7.014 s …  7.083 s    50 runs

Summary
  'Implementation Defined' ran
    1.47 ± 0.01 times faster than 'QARMA'

ARCH=arm64 defconfig + KASAN_SW_TAGS + the KUnit tests:

Benchmark 1: QARMA
  Time (mean ± σ):     185.997 s ±  2.778 s    [User: 184.043 s, System: 0.593 s]
  Range (min … max):   182.816 s … 190.463 s    10 runs

Benchmark 2: Implementation Defined
  Time (mean ± σ):     29.618 s ±  0.301 s    [User: 26.951 s, System: 0.500 s]
  Range (min … max):   29.185 s … 30.103 s    10 runs

Summary
  'Implementation Defined' ran
    6.28 ± 0.11 times faster than 'QARMA'

This should help avoid weird timeouts in CI, as the VMs can be quite
slow.

Aside from the benchmarks above, this change is visible in dmesg:

[    0.000000] CPU features: detected: Address authentication (architected QARMA5 algorithm)

vs.

[    0.000000] CPU features: detected: Address authentication (IMP DEF algorithm)

Link: https://lore.kernel.org/YlgVa+AP0g4IYvzN@lakrids/
Link: https://gitlab.com/qemu-project/qemu/-/blob/v7.0.0/docs/system/arm/cpu-features.rst

[nickdesaulniers]

interesting find! ⏩ 🏃

[nathanchance]

Thanks for the review as always!