[ASAN] Fix fputs interception for 32-bit macOS

On 32-bit macOS, "_fputs$UNIX2003" is called instead of "fputs" and the
"fgets_fputs" test fails. Apparently previous versions still passed the
test due to the internal implementation calling "strlen", but that does
not seem to be the case with macOS 10.13.3. Fixes r334450.

llvm-svn: 334670