[libc] Enable string to integer conversion functions in the default build

Adds atoi, atol, atoll, strtol, strtoll, strtoul, and strtoull to the
list of entrypoints for Windows and aarch64 linux, as well as moving
them out of the LLVM_LIBC_FULL_BUILD condition for x86_64 linux.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D108477