Remove MinGW builder in preference of MinGW-w64.
Remove MinGW builder in preference of MinGW-w64.

The plain MinGW enviroment does not provide any threading supporting, including
in the C++ STL. The MinGW-w64 enviroment does not have this problem.

This patch removes the 32 bit bot since it's always going to fail.

efcs/benchmark,nickhutchinson/benchmark,mockingbirdnest/benchmark,google/benchmark,nickhutchinson/benchmark,disconnect3d/benchmark,everbase/benchmark,fuchsia-mirror/third_party-benchmark,nickhutchinson/benchmark-cxx03,ntx-/benchmark,everbase/benchmark,nickhutchinson/benchmark-cxx03,efcs/benchmark,google/benchmark,codygray/benchmark,mockingbirdnest/benchmark,ntx-/benchmark,nickhutchinson/benchmark-cxx03,efcs/benchmark,everbase/benchmark,codygray/benchmark,mockingbirdnest/benchmark,ntx-/benchmark,efcs/benchmark,disconnect3d/benchmark,mockingbirdnest/benchmark,fuchsia-mirror/third_party-benchmark,nickhutchinson/benchmark,everbase/benchmark,fuchsia-mirror/third_party-benchmark,disconnect3d/benchmark,google/benchmark,fuchsia-mirror/third_party-benchmark,codygray/benchmark,ntx-/benchmark,nickhutchinson/benchmark,nickhutchinson/benchmark-cxx03,codygray/benchmark,disconnect3d/benchmark