[libc] Disable Death Tests While Hermetic (#77388)

The death test infrastructure seems to depend on operator new, which
isn't currently supported in our hermetic tests. This patch just
disables the death tests in hermetic mode since they only overlap in the
nan tests.

libc/test/UnitTest/HermeticTestUtils.cpp

@@ -104,6 +104,10 @@ void *__dso_handle = nullptr;
 
 } // extern "C"
 
+void *operator new(size_t size) { return malloc(size); }
+
+void *operator new[](size_t size) { return malloc(size); }
+
 void operator delete(void *) {
   // The libc runtime should not use the global delete operator. Hence,
   // we just trap here to catch any such accidental usages.

libc/test/src/math/smoke/CMakeLists.txt

@@ -1214,6 +1214,9 @@ add_fp_unittest(
     libc.include.signal
     libc.src.math.nanf
     libc.src.__support.FPUtil.fp_bits
+  # FIXME: The nan tests currently have death tests, which aren't supported for
+  # hermetic tests.
+  UNIT_TEST_ONLY
 )
 
 add_fp_unittest(
@@ -1227,6 +1230,9 @@ add_fp_unittest(
     libc.include.signal
     libc.src.math.nan
     libc.src.__support.FPUtil.fp_bits
+  # FIXME: The nan tests currently have death tests, which aren't supported for
+  # hermetic tests.
+  UNIT_TEST_ONLY
 )
 
 add_fp_unittest(
@@ -1240,6 +1246,9 @@ add_fp_unittest(
     libc.include.signal
     libc.src.math.nanl
     libc.src.__support.FPUtil.fp_bits
+  # FIXME: The nan tests currently have death tests, which aren't supported for
+  # hermetic tests.
+  UNIT_TEST_ONLY
 )
 
 # FIXME: These tests are currently spurious for NVPTX.