Fix eigh UPLO and zero-size eigh/svd on the CPU

[kapellirohith]

Proposed changes

eigh/eigvalsh read the opposite triangle from what UPLO asks for.
The row-major buffer is handed to LAPACK, which reads it as its transpose,
so UPLO="L" actually used the upper triangle. Only observable when the
two triangles differ:

>>> a = mx.array([[1.0, 999.0], [2.0, 3.0]])
>>> mx.linalg.eigvalsh(a, UPLO="L", stream=mx.cpu)
array([-997.001, 1001], dtype=float32)     # before (upper triangle)
array([-0.236068, 4.23607], dtype=float32) # after, matches numpy

Also fixes crashes for zero-size inputs to eigh and svd: LAPACK
rejects lda = 0 and the batch-count division was by zero.

No behavior change for inputs with consistent triangles (symmetric or
Hermitian), which is why the existing tests didn't catch it.

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)