conditional compilation of gcd(SomeInteger,SomeInteger) in std/math (#23773)

The most specific version of `gcd(int,int)` in `std/math` uses bitwise
comparisons from C compilers, which can't be borrowed on the js platform
in the web browser. Conditional compilation here should fix the issue
for this and downstream libraries such as `std/rationals` when compiling
to browser js as the backend.

---------

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

lib/pure/math.nim

@@ -58,6 +58,7 @@ import std/private/since
                        # of the standard library!
 
 import std/[bitops, fenv]
+import system/countbits_impl
 
 when defined(nimPreviewSlimSystem):
   import std/assertions
@@ -1229,40 +1230,42 @@ func gcd*[T](x, y: T): T =
     swap x, y
   abs x
 
-func gcd*(x, y: SomeInteger): SomeInteger =
-  ## Computes the greatest common (positive) divisor of `x` and `y`,
-  ## using the binary GCD (aka Stein's) algorithm.
-  ##
-  ## **See also:**
-  ## * `gcd func <#gcd,T,T>`_ for a float version
-  ## * `lcm func <#lcm,T,T>`_
-  runnableExamples:
-    doAssert gcd(12, 8) == 4
-    doAssert gcd(17, 63) == 1
-
-  when x is SomeSignedInt:
-    var x = abs(x)
-  else:
-    var x = x
-  when y is SomeSignedInt:
-    var y = abs(y)
-  else:
-    var y = y
-
-  if x == 0:
-    return y
-  if y == 0:
-    return x
-
-  let shift = countTrailingZeroBits(x or y)
-  y = y shr countTrailingZeroBits(y)
-  while x != 0:
-    x = x shr countTrailingZeroBits(x)
-    if y > x:
-      swap y, x
-    x -= y
-  y shl shift
-
+when useBuiltins:
+  ## this func uses bitwise comparisons from C compilers, which are not always available.
+  func gcd*(x, y: SomeInteger): SomeInteger =
+    ## Computes the greatest common (positive) divisor of `x` and `y`,
+    ## using the binary GCD (aka Stein's) algorithm.
+    ##
+    ## **See also:**
+    ## * `gcd func <#gcd,T,T>`_ for a float version
+    ## * `lcm func <#lcm,T,T>`_
+    runnableExamples:
+      doAssert gcd(12, 8) == 4
+      doAssert gcd(17, 63) == 1
+  
+    when x is SomeSignedInt:
+      var x = abs(x)
+    else:
+      var x = x
+    when y is SomeSignedInt:
+      var y = abs(y)
+    else:
+      var y = y
+  
+    if x == 0:
+      return y
+    if y == 0:
+      return x
+  
+    let shift = countTrailingZeroBits(x or y)
+    y = y shr countTrailingZeroBits(y)
+    while x != 0:
+      x = x shr countTrailingZeroBits(x)
+      if y > x:
+        swap y, x
+      x -= y
+    y shl shift
+  
 func gcd*[T](x: openArray[T]): T {.since: (1, 1).} =
   ## Computes the greatest common (positive) divisor of the elements of `x`.
   ##