tests: kernel: timer: jitter_drift: Fix FPU exception with picolibc

[golowanow]

Fix 'Floating point unit device not available' exception at test_jitter_drift_timer_period when picolibc is default libc implementation.

fixes #64333

[ceolin]

I don't think that is the proper fix. If the C library needs it, the C library should be the one enabling it.
Also, this test used to work with soft floating point ...

[keith-packard]

Yup, this isn't needed; the bug should also be fixed by #62882

[golowanow]

I don't think that is the proper fix. If the C library needs it, the C library should be the one enabling it. Also, this test used to work with soft floating point ...

C library might be optimized for code size/speed vs. the test might need always the same floating point calculation result precisely.

[keith-packard]

C library might be optimized for code size/speed vs. the test might need always the same floating point calculation result precisely.

Not in this case; picolibc uses exactly the same code on x86 for the computation. The exception occurred because it was trying to check the rounding mode and that code was (incorrectly) using FPU registers on x86 because that code hadn't been disabled when building without an FPU as all picolibc test environments for x86 had the FPU enabled until Zephyr came along.