Enable a faster backend for OpenFHE

[j2kun]

Cf. #1195 for discussion of the impact. We will need to add OpenMP as a dependency.

[j2kun]

We patch in the openfhe config that would be generated by Cmake here: https://github.com/google/heir/blob/main/bazel/openfhe/add_config_core.patch

First step is figuring out what the right generated header file is to replace that patch, then work through build issues.

@ZenithalHourlyRate do you have a copy of the generated src/core/include/config_core.h that came with your faster build from #1195 ?

[ZenithalHourlyRate]

I was just using openfhe-development, not even openfhe-hexl.

$ cat build/src/core/config_core.h
//==================================================
// DO NOT TOUCH THIS FILE! It is generated by cmake
//==================================================
#ifndef __CMAKE_GENERATED_CONFIG_CORE_H__
#define __CMAKE_GENERATED_CONFIG_CORE_H__

/* #undef WITH_BE2 */
#define WITH_BE4
/* #undef WITH_NOISE_DEBUG */
/* #undef WITH_NTL */
/* #undef WITH_TCM */

#define CKKS_M_FACTOR 1
#define HAVE_INT128 TRUE
#define HAVE_INT64 TRUE
#define MATHBACKEND 4
#define NATIVEINT 64

I think the real problem is these lines

heir/bazel/openfhe/openfhe.BUILD

Lines 28 to 30 in 9031113

	OPENFHE_DEFINES = [
	"MATHBACKEND=2",
	"OMP_NUM_THREADS=1",

I tried to delete the "OMP_NUM_THREADS=1" line but the resulting program is still single threaded. Do not know why there is such limitation, maybe it is related to bazel job control?