This PR fixes the regex patterns used to detect VSX3 and VSX4 CPU features
in the ppc64 Meson build scripts. Previously, the regex used a character
class:

'.*[mcpu=|vsx].*'

This incorrectly matched individual characters instead of the intended
substrings "mcpu=" or "vsx".

Changes made:

• Updated the regex to use a non-capturing group with alternation:

  '.*(?:mcpu=|vsx).*'
  

• No changes were made to feature hierarchy, test code, extra tests, or
  compiler flags.

This ensures that the build system correctly detects VSX3/VSX4 features
on both GCC and Clang compilers on ppc64 platforms.

Before:

• VSX3/VSX4 detection could fail or mis-detect features because of
  improper regex.

After:

• Regex properly matches either "mcpu=" or "vsx" as intended.
• Extra tests and compiler flags remain unchanged.