Fallback to use BitScan(Forward|Reverse) on MingW

Perl has functions that return the positions of the least or most
significant one bit in a word.  gcc has builtins that do these specific
operations, and they get compiled to a single instruction on many
hardware platforms.  Windows instead has various BitScan functions that
do the same thing.

MingW has both gcc and Windows, so it should have access to both the
builtins and to the BitScan functions, but prior to this commit, if we
didn't find gcc builtins on MingW, we wouldn't bother to look for the
BitScan functions.

Reading the code caused me to pause to try to figure out why not use
these BitScan functions, and asked on #irc.  The answer from @xenu was
that on MingW (as opposed to MSVC) they're emulated, and not as
efficient as the gcc builtins.

Then I looked at the code again, and realized we don't try them unless
the gcc builtins aren't available, unlikely these days.  But we should
do something so that people who read this code in the future don't also
end up spending time unnecessarily wondering why we treat MingW
specially.

We could add a comment, but it seems to me better to just not make MingW
a special case.  It's likely that the MingW emulation of the BitScan
functions is better than our further fallback, so I see that as better
and cleaner than special casing MingW.

inline.h

@@ -1401,7 +1401,7 @@ Perl_is_utf8_invariant_string_loc(const U8* const s, STRLEN len, const U8 ** ep)
 #  endif
 #endif
 
-#if defined(_MSC_VER)
+#if defined(WIN32)
 #  include <intrin.h>
 #  pragma intrinsic(_BitScanForward)
 #  pragma intrinsic(_BitScanReverse)
@@ -1443,7 +1443,7 @@ Perl_lsbit_pos64(U64 word)
 
     return (unsigned) PERL_CTZ_64(word);
 
-#  elif U64SIZE == 8 && defined(_WIN64) && defined(_MSC_VER)
+#  elif U64SIZE == 8 && defined(_WIN64)
 #    define PERL_HAS_FAST_GET_LSB_POS64
 
     {
@@ -1497,7 +1497,7 @@ Perl_lsbit_pos32(U32 word)
 
     return (unsigned) PERL_CTZ_32(word);
 
-#elif U32SIZE == 4 && defined(_MSC_VER)
+#elif U32SIZE == 4 && defined(WIN32)
 #  define PERL_HAS_FAST_GET_LSB_POS32
 
     {
@@ -1551,7 +1551,7 @@ Perl_msbit_pos64(U64 word)
 
     return (unsigned) LZC_TO_MSBIT_POS_(U64, PERL_CLZ_64(word));
 
-#  elif U64SIZE == 8 && defined(_WIN64) && defined(_MSC_VER)
+#  elif U64SIZE == 8 && defined(_WIN64)
 #    define PERL_HAS_FAST_GET_MSB_POS64
 
     {
@@ -1607,8 +1607,7 @@ Perl_msbit_pos32(U32 word)
 #  define PERL_HAS_FAST_GET_MSB_POS32
 
     return (unsigned) LZC_TO_MSBIT_POS_(U32, PERL_CLZ_32(word));
-
-#elif U32SIZE == 4 && defined(_MSC_VER)
+#elif U32SIZE == 4 && defined(WIN32)
 #  define PERL_HAS_FAST_GET_MSB_POS32
 
     {