cpu.x86: enable POPCNT with a flag

If bootstrapped with '-enable-popcnt' and the host CPU supports it, enable the POPCNT-based fixnum-bit-count intrinsic. Fixes #436.

basis/cpu/x86/32/32.factor

@@ -236,4 +236,4 @@ M: x86.32 flatten-struct-type
 
 M: x86.32 struct-return-on-stack? os linux? not ;
 
-check-sse
+check-cpu-features

basis/cpu/x86/64/64.factor

@@ -150,4 +150,4 @@ USE: vocabs
     { [ os windows? ] [ "cpu.x86.64.windows" require ] }
 } cond
 
-check-sse
+check-cpu-features

basis/cpu/x86/features/features.factor

@@ -77,8 +77,6 @@ PRIVATE>
 MEMO: sse-version ( -- n )
     (sse-version) "sse-version" get string>number [ min ] when* ;
 
-[ \ sse-version reset-memoized ] "cpu.x86.features" add-startup-hook
-
 : sse? ( -- ? ) sse-version 10 >= ;
 : sse2? ( -- ? ) sse-version 20 >= ;
 : sse3? ( -- ? ) sse-version 30 >= ;
@@ -95,6 +93,12 @@ MEMO: sse-version ( -- n )
         return-reg SETB
     ] alien-assembly ;
 
+MEMO: enable-popcnt? ( -- ? )
+    popcnt? "enable-popcnt" get and ;
+
+[ { sse-version enable-popcnt? } [ reset-memoized ] each ]
+"cpu.x86.features" add-startup-hook
+
 : sse-string ( version -- string )
     {
         { 00 [ "no SSE" ] }

basis/cpu/x86/x86.factor

@@ -776,7 +776,17 @@ enable-log2
 
 : check-sse ( -- )
     "Checking for multimedia extensions... " write flush
-    [ { (sse-version) } compile ] with-optimizer
     sse-version
     [ sse-string " detected" append print ]
     [ 20 < "cpu.x86.x87" "cpu.x86.sse" ? require ] bi ;
+
+: check-popcnt ( -- )
+    enable-popcnt? [
+        "Building with POPCNT support" print
+        enable-bit-count
+    ] when ;
+
+: check-cpu-features ( -- )
+    [ { (sse-version) popcnt? } compile ] with-optimizer
+    check-sse
+    check-popcnt ;