Introduce getenv to LLVM libc

Add support for getenv as defined by the Open Group's "System Interface &
 Header" in https://pubs.opengroup.org/onlinepubs/7908799/xsh/getenv.html



getenv requires a standard way of accessing the environment,
so a pointer to the environment is added to the startup in crt1.
Consquently, this function is not usable on top of other libcs.

Added starts_with method to StringView. getenv function uses it.

Co-authored-by: Jeff Bailey <jeffbailey@google.com>

Reviewed By: sivachandra, rtenneti

Differential Revision: https://reviews.llvm.org/D119403