[sanitizer] Fix comment (NFC)

As pointed out in D85387, part of the comment for MapDynamicShadow
refactored to sanitizer_common in D83247 was incorrect for non-Linux
versions. Update the comment to reflect that.

Author: teresajohnson

compiler-rt/lib/sanitizer_common/sanitizer_common.h

@@ -130,7 +130,7 @@ void UnmapFromTo(uptr from, uptr to);
 // be aligned to the mmap granularity * 2^shadow_scale, or to
 // 2^min_shadow_base_alignment if that is larger. The returned address will
 // have max(2^min_shadow_base_alignment, mmap granularity) on the left, and
-// shadow_size_bytes bytes on the right, mapped no access.
+// shadow_size_bytes bytes on the right, which on linux is mapped no access.
 // The high_mem_end may be updated if the original shadow size doesn't fit.
 uptr MapDynamicShadow(uptr shadow_size_bytes, uptr shadow_scale,
                       uptr min_shadow_base_alignment, uptr &high_mem_end);