Add work-arounds for missing C++11 features.

Add work-arounds for missing C++11 features.

Android versions before M did not have:

std::shared_ptr
std::unique_ptr
std::to_string

This is fixed by extending stlport with custom implementations for
these features. This works by using 'string' and 'memory' header
files in a path searched before stlport itself, in which the new
features are implemented, and including stport's original headers for
all other functionality.

Bug b/75229322

Change-Id: Icd05c072c366381155e086c3f14f805bc4f104d6
Reviewed-on: https://swiftshader-review.googlesource.com/18408
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>