Update libretro-common to latest version

[JoeMatt]

Problem

The vendored libretro-common code was last updated in October 2021 (~4.5 years ago). This means we're missing:

• Bug fixes and stability improvements
• New API support (e.g., newer core options API versions)
• Platform compatibility improvements (newer compilers, newer OS versions)
• Security fixes

Proposal

Update the vendored libretro-common/ directory to the latest version from https://github.com/libretro/libretro-common.

Notes

• Only the files we actually use need to be updated (see Makefile.common for the list)
• Need to verify our #include usage still matches the updated API
• Should be tested across all CI platforms (Linux GCC, macOS Clang)

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