AIX: Fix the type of the 4th arguement of getgrnam_r()

[xingxue-ibm]

Description

This PR changes the type of the buflen argument of the getgrnam_r() routine from c_int to size_t to align with the POSIX specification.

POSIX:

int getgrnam_r(const char *name, struct group *grp, char *buffer, size_t bufsize, struct group **result);

Sources

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[tgross35]

Thanks!

[xingxue-ibm]

@rustbot label +stable-nominated