Fix fp16 related testcase failure for i686.

> I see some regressions most likely with this change on i686-linux,
> in particular:
> +FAIL: gcc.dg/pr107547.c (test for excess errors)
> +FAIL: gcc.dg/torture/floatn-convert.c  -O0 (test for excess errors)
> +UNRESOLVED: gcc.dg/torture/floatn-convert.c  -O0 compilation failed to produce executable
> +FAIL: gcc.dg/torture/floatn-convert.c  -O1 (test for excess errors)
> +UNRESOLVED: gcc.dg/torture/floatn-convert.c  -O1 compilation failed to produce executable
> +FAIL: gcc.dg/torture/floatn-convert.c  -O2 (test for excess errors)
> +UNRESOLVED: gcc.dg/torture/floatn-convert.c  -O2 compilation failed to produce executable
> +FAIL: gcc.dg/torture/floatn-convert.c  -O2 -flto (test for excess errors)
> +UNRESOLVED: gcc.dg/torture/floatn-convert.c  -O2 -flto compilation failed to produce executable
> +FAIL: gcc.dg/torture/floatn-convert.c  -O2 -flto -flto-partition=none (test for excess errors)
> +UNRESOLVED: gcc.dg/torture/floatn-convert.c  -O2 -flto -flto-partition=none compilation failed to produce executable
> +FAIL: gcc.dg/torture/floatn-convert.c  -O3 -fomit-frame-pointer -funroll-loops -fpeel-loops -ftracer -finline-functions (test for excess errors)
> +UNRESOLVED: gcc.dg/torture/floatn-convert.c  -O3 -fomit-frame-pointer -funroll-loops -fpeel-loops -ftracer -finline-functions compilation failed to produce executable
> +FAIL: gcc.dg/torture/floatn-convert.c  -O3 -g (test for excess errors)
> +UNRESOLVED: gcc.dg/torture/floatn-convert.c  -O3 -g compilation failed to produce executable
> +FAIL: gcc.dg/torture/floatn-convert.c  -Os (test for excess errors)
> +UNRESOLVED: gcc.dg/torture/floatn-convert.c  -Os compilation failed to produce executable
> +FAIL: gcc.target/i386/float16-7.c (test for errors, line 7)
>

> Perhaps we need to tweak
> gcc/testsuite/lib/target-supports.exp (add_options_for_float16)
> so that it adds -msse2 for i?86-*-* x86_64-*-* (that would likely
> fix up floatn-convert) and for the others perhaps
> /* { dg-add-options float16 } */
> ?

I've verified the change fixed those failures.
Ready to push to trunk if there's no objections.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr107547.c: Add { dg-add-options float16 }.
	* gcc.target/i386/float16-7.c: Add -msse2 to dg-options.
	* lib/target-supports.exp (add_options_for_float16): Add
	-msse2 for i?86-*-* || x86_64-*-*.

gcc/testsuite/gcc.dg/pr107547.c

@@ -1,6 +1,7 @@
 /* PR tree-optimization/107547 */
 /* { dg-do compile } */
 /* { dg-options "-O2" } */
+/* { dg-add-options float16 } */
 
 int x;
 

gcc/testsuite/gcc.target/i386/float16-7.c

@@ -1,5 +1,5 @@
 /* { dg-do compile } */
-/* { dg-options "-O2 -mfpmath=387 -fexcess-precision=16" } */
+/* { dg-options "-O2 -msse2 -mfpmath=387 -fexcess-precision=16" } */
 /* { dg-excess-errors "'-fexcess-precision=16' is not compatible with '-mfpmath=387'" } */
 _Float16
 foo (_Float16 a, _Float16 b)

gcc/testsuite/lib/target-supports.exp

@@ -3426,6 +3426,9 @@ proc add_options_for_float16 { flags } {
     if { [istarget arm*-*-*] } {
 	return "$flags -mfp16-format=ieee"
     }
+    if { [istarget i?86-*-*] || [istarget x86_64-*-*] } {
+	return "$flags -msse2"
+    }
     return "$flags"
 }