tests: correctness/float16_t: mark __extendhfsf2 with default visib…

…ility (halide#8084)

```
[2336/4154] /usr/bin/clang++-17 -DHALIDE_ENABLE_RTTI -DHALIDE_VERSION_MAJOR=17 -DHALIDE_VERSION_MINOR=0 -DHALIDE_VERSION_PATCH=0 -DHALIDE_WITH_EXCEPTIONS -I/build/halide-17.0.0/test/common -I/build/halide-17.0.0/tools -I/build/halide-17.0.0/build/stage-1/halide/include -g -fdebug-default-version=4 -fprofile-use=/build/halide-17.0.0/build-profile/default.profdata -fcs-profile-generate -Xclang -mllvm -Xclang -vp-counters-per-site=100.0 -fuse-ld=lld-17 -Wl,--build-id=sha1 -std=c++17 -flto=thin -fPIE -fvisibility=hidden -fvisibility-inlines-hidden -Winvalid-pch -Xclang -include-pch -Xclang /build/halide-17.0.0/build/stage-1/halide/test/CMakeFiles/_test_internal.dir/cmake_pch.hxx.pch -Xclang -include -Xclang /build/halide-17.0.0/build/stage-1/halide/test/CMakeFiles/_test_internal.dir/cmake_pch.hxx -MD -MT test/correctness/CMakeFiles/correctness_float16_t.dir/float16_t.cpp.o -MF test/correctness/CMakeFiles/correctness_float16_t.dir/float16_t.cpp.o.d -o test/correctness/CMakeFiles/correctness_float16_t.dir/float16_t.cpp.o -c /build/halide-17.0.0/test/correctness/float16_t.cpp
<...>
ld.lld-17: error: undefined hidden symbol: __extendhfsf2
>>> referenced by float16_t.cpp:391 (/build/halide-17.0.0/test/correctness/float16_t.cpp:391)
>>>               lto.tmp:(main)
>>> did you mean: __extendbfsf2
>>> defined in: /lib/x86_64-linux-gnu/libgcc_s.so.1
clang++-17: error: linker command failed with exit code 1 (use -v to see invocation)

```

test/correctness/float16_t.cpp

@@ -12,15 +12,15 @@ extern "C" {
 
 // In Clang 15 and later, this function is passed a uint16... but in the xmm0 register on x86-64.
 // So we'll declare it as a float and just grab the upper 16 bits.
-__attribute__((weak)) float __extendhfsf2(float actually_a_float16) {
+__attribute__((weak, visibility("default"))) float __extendhfsf2(float actually_a_float16) {
     uint16_t data;
     memcpy(&data, &actually_a_float16, sizeof(data));
     return (float)Halide::float16_t::make_from_bits(data);
 }
 
 #else
 
-__attribute__((weak)) float __extendhfsf2(uint16_t data) {
+__attribute__((weak, visibility("default"))) float __extendhfsf2(uint16_t data) {
     return (float)Halide::float16_t::make_from_bits(data);
 }