cmake : Handle mixed-case 'Power' strings in POWER CPU detection (llama/13966)

cmake : Handle mixed-case 'Power' strings in POWER CPU detection (llama/13966)

Some systems report the CPU implementation as "Power11" instead of "POWER11".
The existing CMake logic uses a case-sensitive regular expression to extract
the CPU generation, which fails when the casing doesn't exactly match "POWER".

This patch provides a fix by first converting the string to uppercase before applying the regex.

Signed-off-by: root <redacted>
Co-authored-by: root <redacted>