AVX2 with GCC9 fix. (#18991)

AVX2 with GCC9 fix. (#18991)

Summary:
Dear All,

The proposed patch fixes the test code snippets used in cmake infrastructure, and implicit failure to set properly the ```CAFFE2_COMPILER_SUPPORTS_AVX2_EXTENSIONS``` flag. The libcaffe2.so will have some ```UND``` avx2 related references, rendering it unusable.

* Using GCC 9 test code from cmake build infra always fails:
```
$ gcc  -O2 -g -pipe -Wall -m64 -mtune=generic -fopenmp -DCXX_HAS_AVX_1 -fPIE -o test.o -c test.c -mavx2
test.c: In function ‘main’:
test.c:11:26: error: incompatible type for argument 1 of ‘_mm256_extract_epi64’
   11 |     _mm256_extract_epi64(x, 0); // we rely on this in our AVX2 code
      |                          ^
      |                          |
      |                          __m256 {aka __vector(8) float}
In file included from /usr/lib/gcc/x86_64-redhat-linux/9/include/immintrin.h:51,
                 from test.c:4:
/usr/lib/gcc/x86_64-redhat-linux/9/include/avxintrin.h:550:31: note: expected ‘__m256i’ {aka ‘__vector(4) long long int’} but argument is of type ‘__m256’ {aka ‘__vector(8) float’}
  550 | _mm256_extract_epi64 (__m256i __X, const int __N)
      |

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/9/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-redhat-linux
Configured with: ../configure --enable-bootstrap --enable-languages=c,c++,fortran,objc,obj-c++,ada,go,d,lto --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared --enable-threads=posix --enable-checking=release --enable-multilib --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --with-gcc-major-version-only --with-linker-hash-style=gnu --enable-plugin --enable-initfini-array --with-isl --enable-offload-targets=nvptx-none --without-cuda-driver --enable-gnu-indirect-function --enable-cet --with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
Thread model: posix
gcc version 9.0.1 20190328 (Red Hat 9.0.1-0.12) (GCC)
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18991

Differential Revision: D14821838

Pulled By: ezyang

fbshipit-source-id: 7eb3a854a1a831f6fda8ed7ad089746230b529d7