graphics/svgalib: unbreak with clang 4.0

In file included from gl/grlib.c:7:
gl/inlstring.h:71:9: error: asm-specifier for input or output variable conflicts with asm clobber list

:         "cx", "di");
          ^

gl/inlstring.h:281:17: error: asm-specifier for input or output variable conflicts with asm clobber list

:"ax", "0", "1", "2");
            ^

gl/inlstring.h:297:17: error: asm-specifier for input or output variable conflicts with asm clobber list

:"ax", "0", "1", "2");
            ^

gl/inlstring.h:314:17: error: asm-specifier for input or output variable conflicts with asm clobber list

:"ax", "0", "1", "2");
            ^

PR: 216154
Reported by: antoine (via exp-run)
Submitted by: dim
Approved by: portmgr blanket