[X86] Remove __builtin_ia32_pabs intrinsics and use generic __builtin_elementwise_abs

D111986 added the generic `__builtin_elementwise_abs()` intrinsic with the same integer absolute behaviour as the SSE/AVX instructions (abs(INT_MIN) == INT_MIN)

This patch removes the `__builtin_ia32_pabs*` intrinsics and just uses `__builtin_elementwise_abs` - the existing tests see no changes:
```
__m256i test_mm256_abs_epi8(__m256i a) {
  // CHECK-LABEL: test_mm256_abs_epi8
  // CHECK: [[ABS:%.*]] = call <32 x i8> @llvm.abs.v32i8(<32 x i8> %{{.*}}, i1 false)
  return _mm256_abs_epi8(a);
}
```
This requires us to add a `__v64qs` explicitly signed char vector type (we already have `__v16qs` and `__v32qs`).

Differential Revision: https://reviews.llvm.org/D117791