ext/bcmath: Improving mul performance 2

[SakiTakamachi]

This is derived from #14213.
Also, I am considering using "Karatsuba-algorithm", but I will separate it into another PR.

bench codes:

1:

$num1 = '1.2345678';
$num2 = '2.1234567';

for ($i = 0; $i < 5000000; $i++) {
    bcmul($num1, $num2, 7);
}

2:

$num1 = str_repeat('1234567890', 300);
$num2 = str_repeat('9876543210', 300);

for ($i = 0; $i < 6000; $i++) {
    bcmul($num1, $num2, 0);
}

3:

$num1 = '1.2345678901234567890';
$num2 = '2.12345678901234567890';

for ($i = 0; $i < 3000000; $i++) {
    bcmul($num1, $num2, 20);
}

4:

$num1 = '-12345678901234567890.12345678901234567890';
$num2 = '-212345678901234567890.12345678901234567890';

for ($i = 0; $i < 1800000; $i++) {
    bcmul($num1, $num2, 20);
}

5:

$num1 = '12345678901234567890.00000000000000000000000000000000000000000000000000';
$num2 = '212345678901234567890.00000000000000000000000000000000000000000000000000';

for ($i = 0; $i < 3000000; $i++) {
    bcmul($num1, $num2, 20);
}

before

hyperfine "php /bc/mul/1.php" --warmup 10
Time (mean ± σ):     620.9 ms ±   6.1 ms    [User: 615.4 ms, System: 4.6 ms]
Range (min … max):   612.6 ms … 632.7 ms    10 runs

hyperfine "php /bc/mul/2.php" --warmup 10
Time (mean ± σ):     581.2 ms ±   6.0 ms    [User: 578.2 ms, System: 2.3 ms]
Range (min … max):   571.5 ms … 590.9 ms    10 runs

hyperfine "php /bc/mul/3.php" --warmup 10
Time (mean ± σ):     628.7 ms ±  13.6 ms    [User: 624.9 ms, System: 3.0 ms]
Range (min … max):   614.6 ms … 656.8 ms    10 runs

hyperfine "php /bc/mul/4.php" --warmup 10
Time (mean ± σ):     593.6 ms ±  10.1 ms    [User: 589.6 ms, System: 3.1 ms]
Range (min … max):   583.7 ms … 615.4 ms    10 runs

hyperfine "php /bc/mul/5.php" --warmup 10
Time (mean ± σ):     680.9 ms ±   8.6 ms    [User: 676.8 ms, System: 3.2 ms]
Range (min … max):   671.1 ms … 694.5 ms    10 runs

after

hyperfine "php /bc/mul/1.php" --warmup 10
Time (mean ± σ):     604.5 ms ±   8.7 ms    [User: 600.0 ms, System: 3.5 ms]
Range (min … max):   596.1 ms … 625.3 ms    10 runs

hyperfine "php /bc/mul/2.php" --warmup 10
Time (mean ± σ):     541.1 ms ±   9.0 ms    [User: 537.9 ms, System: 2.4 ms]
Range (min … max):   534.1 ms … 565.5 ms    10 runs

hyperfine "php /bc/mul/3.php" --warmup 10
Time (mean ± σ):     506.8 ms ±   2.9 ms    [User: 502.6 ms, System: 3.3 ms]
Range (min … max):   501.7 ms … 512.0 ms    10 runs

hyperfine "php /bc/mul/4.php" --warmup 10
Time (mean ± σ):     448.6 ms ±   8.1 ms    [User: 444.9 ms, System: 2.9 ms]
Range (min … max):   441.5 ms … 467.7 ms    10 runs

hyperfine "php /bc/mul/5.php" --warmup 10
Time (mean ± σ):     561.4 ms ±   4.4 ms    [User: 557.7 ms, System: 3.0 ms]
Range (min … max):   556.7 ms … 570.7 ms    10 runs

[SakiTakamachi]

I made it a draft because the original hasn't been merged yet.