Run the asinh(+0) test with LLVM libc when the SDK provides asinh

LLVM libc's math.h doesn't declare asinh, so declare it and only skip the test when the compiler's
implementation is used.

test/pico_float_test/pico_double_test.c

diff --git a/test/pico_float_test/pico_double_test.c b/test/pico_float_test/pico_double_test.c
index 752b3e6..f5bbd06 100644
--- a/test/pico_float_test/pico_double_test.c
+++ b/test/pico_float_test/pico_double_test.c
@@ -27,6 +27,11 @@
 #include <sys/types.h>
 #include "inttypes.h"
 
+#if defined(__LLVM_LIBC__) && defined(__llvm__)
+// LLVM libc doesn't declare asinh, but the SDK implementation provides it
+double asinh(double);
+#endif
+
 #if !LIB_PICO_DOUBLE_COMPILER
 #define TEST_SATURATION 1
 #endif
@@ -362,7 +367,7 @@
 
     bool fail = false;
 
-#if !(defined(__LLVM_LIBC__) && defined(__llvm__))
+#if !(defined(__LLVM_LIBC__) && defined(__llvm__) && LIB_PICO_DOUBLE_COMPILER)
     // asinh(+0) must keep its sign
     {
         volatile double zero = 0.0;