swr: Fix build with GCC 10.

swr: Fix build with GCC 10.

GCC 10 added _mm256_storeu2_m128i.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91341

This patch fixes this build error with GCC 10.

In file included from src/gallium/drivers/swr/rasterizer/codegen/gen_knobs.cpp:39:
../src/gallium/drivers/swr/rasterizer/common/os.h:178:20: error: ‘void _mm256_storeu2_m128i(__m128i*, __m128i*, __m256i)’ redeclared inline without ‘gnu_inline’ attribute
  178 | static INLINE void _mm256_storeu2_m128i(__m128i* hi, __m128i* lo, __m256i a)
      |                    ^~~~~~~~~~~~~~~~~~~~
In file included from /usr/lib/gcc/x86_64-redhat-linux/10/include/immintrin.h:51,
                 from /usr/lib/gcc/x86_64-redhat-linux/10/include/x86intrin.h:32,
                 from ../src/gallium/drivers/swr/rasterizer/common/os.h:107,
                 from src/gallium/drivers/swr/rasterizer/codegen/gen_knobs.cpp:39:
/usr/lib/gcc/x86_64-redhat-linux/10/include/avxintrin.h:1580:1: note: ‘void _mm256_storeu2_m128i(__m128i_u*, __m128i_u*, __m256i)’ previously defined here
 1580 | _mm256_storeu2_m128i (__m128i_u *__PH, __m128i_u *__PL, __m256i __A)
      | ^~~~~~~~~~~~~~~~~~~~

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3650>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3650>