[libc] Fix readlink tests on 32-bit systems (#98168)
This patch removes the call sizeof(<Cstring>) and makes buf a
fixed-length array.
The test case worked in 64-bit systems because the sizeof(CString) was
big enough (19 bytes) to hold the contents of FILENAME, but in 32-bit
systems sizeof(CString) is only 12 bytes long, causing a truncation of
FILENAME when placing it in buf.
