libstdc++: Mark gencanon_eng_neg test as unsupported if __float128 is unavailable.
The gencanon_eng_neg test is marked as unsupported when __float128 is not available, preventing errors.
The gencanon_eng_neg test in libstdc++ requires the __float128 type. This commit ensures that the test is only run on targets that support __float128, preventing errors on other platforms by adding dg-require-effective-target to the test.
In Details
The gencanon_eng_neg test relies on __float128, a 128-bit floating-point type. The testsuite uses DejaGnu, which reads all lines in a test file. The dg-require-effective-target directive ensures that the test is only executed if the specified target feature is available.
For Context
In C++, a test suite validates that the library functions as expected across various platforms. __float128 is a data type representing a 128-bit floating-point number, offering higher precision than double. This commit modifies a test case to ensure it only executes on systems that provide support for __float128, avoiding spurious errors in the test results.