[libc] replace for loops with a call to memcpy in File (#165219)

Addresses `TODO`s in file.cpp by replacing data copies via for loops
with calls to inline_memcpy.

Signed-off-by: Shreeyash Pandey <shreeyash335@gmail.com>
GitOrigin-RevId: bbece4b78b073a2377a74158f6b33d164df4d179