file_path: avoid fortified realpath() buffer size abort

[s3gfaultx]

Guidelines

1. Rebase before opening a pull request
2. If you are sending several unrelated fixes or features, use a branch and a separate pull request for each
3. If possible try squashing everything in a single commit. This is particularly beneficial in the case of feature merges since it allows easy bisecting when a problem arises
4. RetroArch codebase follows C89 coding rules for portability across many old platforms check using C89_BUILD=1

Description

Some distributions now build packages with stricter default toolchain hardening, including _FORTIFY_SOURCE=3. Under those settings, glibc’s fortified realpath() performs a compile/runtime object-size check on the destination buffer and aborts if it is smaller than the system PATH_MAX.

glibc's fortified realpath() checks that the destination buffer is at least PATH_MAX bytes. path_resolve_realpath() was passing the caller's buffer directly, which may be smaller than the system PATH_MAX despite being sized to RetroArch's PATH_MAX_LENGTH.

With _FORTIFY_SOURCE=3 this can abort in __realpath_chk when resolving core updater paths.

Resolve into an allocated realpath() buffer instead, then copy the result back into the caller-provided buffer using the provided length.