udpspeeder: build with the toolchain CXX and flags

udpspeeder: build with the toolchain CXX and flags

Patch the upstream makefile to build the cross target with $(CXX) and the
toolchain $(CXXFLAGS)/$(LDFLAGS), and take the version from an overridable
variable. This replaces the Build/Prepare sed rewrites, and the binary now
picks up the hardening flags.

Honouring CXXFLAGS turns on -mips16 for the mips_24kc targets, where the
C++ atomics compile to a sync instruction MIPS16 cannot encode, so build
them with no-mips16.

Signed-off-by: David Connolly <redacted>