gallium/util: Use GCC built-in functions for NaN and infinity.

This patch fixes this build failure with Intel Compiler.

src/gallium/auxiliary/util/u_format_tests.c(903): error: floating-point operation result is out of range
     {PIPE_FORMAT_R16_FLOAT, PACKED_1x16(0xffff), PACKED_1x16(0x7c01), UNPACKED_1x1(        NAN, 0.0, 0.0, 1.0)},

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: José Fonseca <jfonseca@vmware.com>

src/gallium/auxiliary/util/u_format_tests.c

@@ -66,8 +66,13 @@
        {{ 0,  0,  0,  0}, { 0,  0,  0,  0}, {0, 0, 0, 0}, {0, 0, 0, 0}}}
 
 
+#ifdef __GNUC__
+#define NAN __builtin_nan("")
+#define INF __builtin_inf()
+#else
 #define NAN (0.0 / 0.0)
 #define INF (1.0 / 0.0)
+#endif
 
 /**
  * Test cases.