[OpenMP] Use -qarch=auto with XL compiler

CHANGELOG.md

@@ -15,7 +15,8 @@ All notable changes to this project will be documented in this file.
 - Output formatting of Kokkos implementation.
 - Capitalisation of Kokkos filenames.
 - Updated HIP implementation to new interface.
-- Use parallel loop instead of kernels for OpenACC
+- Use parallel loop instead of kernels for OpenACC.
+- OpenMP build for XL compiler uses `-qarch=auto`.
 
 ### Removed
 - Superfluous OpenMP 4.5 map(to:) clauses on kernel target regions.

OpenMP.make

@@ -34,7 +34,7 @@ FLAGS_GNU = -O3 -std=c++11 -mcpu=native
 FLAGS_INTEL = -O3 -std=c++11 -xHOST -qopt-streaming-stores=always
 FLAGS_CRAY = -O3 -hstd=c++11
 FLAGS_CLANG = -O3 -std=c++11
-FLAGS_XL = -O5 -qarch=pwr8 -qtune=pwr8 -std=c++11
+FLAGS_XL = -O5 -qarch=auto -qtune=auto -std=c++11
 FLAGS_PGI = -O3 -std=c++11
 FLAGS_NEC = -O4 -finline -std=c++11
 CXXFLAGS = $(FLAGS_$(COMPILER))