[libc][x86-64] missing entrypoints' definitions in full build mode

[SchrodingerZhu]

Recent patch lands that the <math.h> inclusion is replaced by "hdr/math_macros.h". However, as the name suggestions, the new proxy header does not provide correct definitions for entrypoints. They should be included separately. This leads to the following unhappiness of our build bots:

/home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-fullbuild-dbg/llvm-project/libc/test/utils/FPUtil/x86_long_double_test.cpp:31:32: error: use of undeclared identifier 'isnan'
    ASSERT_NE(static_cast<int>(isnan(nan)), 0);
                               ^
/home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-fullbuild-dbg/llvm-project/libc/test/utils/FPUtil/x86_long_double_test.cpp:42:32: error: use of undeclared identifier 'isnan'
    ASSERT_NE(static_cast<int>(isnan(nan)), 0);
                               ^
/home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-fullbuild-dbg/llvm-project/libc/test/utils/FPUtil/x86_long_double_test.cpp:53:32: error: use of undeclared identifier 'isnan'
    ASSERT_NE(static_cast<int>(isnan(nan)), 0);
                               ^
/home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-fullbuild-dbg/llvm-project/libc/test/utils/FPUtil/x86_long_double_test.cpp:64:32: error: use of undeclared identifier 'isnan'
    ASSERT_EQ(static_cast<int>(isnan(valid)), 0);
                               ^
/home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-fullbuild-dbg/llvm-project/libc/test/utils/FPUtil/x86_long_double_test.cpp:74:32: error: use of undeclared identifier 'isnan'
    ASSERT_EQ(static_cast<int>(isnan(valid)), 0);
                               ^
/home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-fullbuild-dbg/llvm-project/libc/test/utils/FPUtil/x86_long_double_test.cpp:84:32: error: use of undeclared identifier 'isnan'
    ASSERT_EQ(static_cast<int>(isnan(valid)), 0);
                               ^