[libc] add strtoll function and backend

This change adds the stroll function, but most of the implementation is
in the new file str_conv_utils.h since many of the other integer
conversion functions are implemented through what are effectively calls
to strtoll.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D107792

GitOrigin-RevId: b062d639bb3a315f2b5f410e99b9c3ea10c530cc
10 files changed