A recent call in the HORIZON 2020 program includes options specifically mentioning mathematics. Within the Future and Emerging Technologies (FET) branch, one of the subtopics specifically mentioned under “Transition to Exascale Computing”  (FET HPC 02-2017) is: Mathematics and algorithms for extreme scale HPC systems and applications working with extreme data. Applications for such a Research and Innovation Action (RIA) are solicited and will, if funded, receive 2-4 M€. Another option arises under the heading “Exascale HPC ecosystem development (FET HPC 03-2017), specifically the Coordinated Support Action (CSA) Coordination of the Exascale HPC strategy and International Collaboration.  A successful application will be funded with 1-2 M€. The deadline for both calls is 26 September 2017. 
Specific descriptions of the area Mathematics and Algorithms for extreme scale HPC systems can be found in chapter 5.7 (pp. 47 ff) of this agenda document.
The initiative EU-MATHS-IN has volunteered coordinating appropriate actions.