libc: omit memcpy, memmove and memset

LLVM provides these functions as intrinsics, and will generate calls to
libc when appropriate. They are exposed in the `ptr` module as
`copy_nonoverlapping_memory`, `copy_memory` and `set_memory`.
auto merge of #6979 : thestinger/rust/libc, r=brson

LLVM provides these functions as intrinsics, and will generate calls to
libc when appropriate. They are exposed in the `ptr` module as
`copy_nonoverlapping_memory`, `copy_memory` and `set_memory`.

@graydon: r?