Remove non-ASCII characters from C/C++ files; drop /utf-8 on MSVC.

Only the newest versions of MSVC 2015's and MSVC 2017's cl.exe support
the `/utf-8` flag. MSVC 2013 and even the initial versions of MSVC 2015
didn't support it. Make it a non-issue by removing the single non-ASCII
character in C/C++ files.

build.rs

@@ -287,7 +287,6 @@ fn cpp_flags(target: &Target) -> &'static [&'static str] {
             "/Zc:forScope",
             "/Zc:inline",
             "/Zc:rvalueCast",
-            "/utf-8", // Input files are Unicode.
 
             // Warnings.
             "/sdl",

crypto/bn/montgomery_inv.c

@@ -127,7 +127,7 @@ static uint64_t bn_neg_inv_mod_r_u64(uint64_t n) {
 
     /* The addition can overflow, so use Dietz's method for it.
      *
-     * Dietz calculates (x+y)/2 by (x⊕y)>>1 + x&y. This is valid for all
+     * Dietz calculates (x+y)/2 by (x xor y)>>1 + x&y. This is valid for all
      * (unsigned) x and y, even when x+y overflows. Evidence for 32-bit values
      * (embedded in 64 bits to so that overflow can be ignored):
      *