libgcc_s: only export 128-bit long double when available

These functions are only available on aarch64 and riscv so only try to
export them on those architectures.

Differential Revision:	https://reviews.freebsd.org/D44322

lib/libgcc_s/Symbol.map

@@ -148,6 +148,7 @@ GCC_4.3.0 {
 	__bswapsi2;
 };
 
+#if defined(__aarch64__) || defined(__riscv)
 GCC_4.6.0 {
 	__addtf3;
 	__cmptf2;
@@ -180,3 +181,4 @@ GCC_4.6.0 {
 	__trunctfxf2;
 	__unordtf2;
 };
+#endif