Allow dcurl to compile and execute AVX code implementation on the hardware not supporting AVX2

[marktwtn]

The BUILD_AVX=1 command option would enable AVX code implementation of dcurl.
However, the current AVX code has two different implementations.

• Support AVX instruction.
  Not the default. Need to modify Makefile and the source code to use it.
• Support AVX and AVX2 instruction.
  The default.

The issue is focused on using the correct AVX code implementation automatically based on the hardware supported instruction.

[jserv]

AVX (not AVX2) support is crucial to AMD Ryzen since its AVX2 is known to be slower than Intel Core i9 series.

[marktwtn]

The -Ofast optimization level makes AVX version unable to finish the execution.
However, it does not happen on AVX2 version.
If we change the optimization level to -O3, the problem would disappear.

The GCC version has nothing to do with the problem.

The difference of the assembly code:

-O3

        vucomisd        %xmm3, %xmm0
        jp      .L114
        jne     .L114

-Ofast

        vcomisd %xmm3, %xmm0
        jne     .L114

These code happens when the __m256d type variable is compared to another constant value or uses its value in the logical operation.

Example:

__m256d nonce_probe = ...
...
nonce_probe[0] == LBITS

__m256d carry;
...
i == INCR_START || carry[0]

The jp instruction would jump if one of the previous comparison operand is NaN.
The NaN value are defined in the IEEE floating-point standard.

The bitwise operation in the PoW might created the NaN value.
However, the NaN value is not handled in the -Ofast optimization level since it optimizes out the jp instruction.

[jserv]

Let's stick to -O3 optimization order and explain for further tracking.