Force DT_RPATH for --enable-hardcoded-path-in-tests

On Fedora 40/x86-64, linker enables --enable-new-dtags by default which
generates DT_RUNPATH instead of DT_RPATH.  Unlike DT_RPATH, DT_RUNPATH
only applies to DT_NEEDED entries in the executable and doesn't applies
to DT_NEEDED entries in shared libraries which are loaded via DT_NEEDED
entries in the executable.  Some glibc tests have libstdc++.so.6 in
DT_NEEDED, which has libm.so.6 in DT_NEEDED.  When DT_RUNPATH is generated,
/lib64/libm.so.6 is loaded for such tests.  If the newly built glibc is
older than glibc 2.36, these tests fail with

assert/tst-assert-c++: /export/build/gnu/tools-build/glibc-gitlab-release/build-x86_64-linux/libc.so.6: version `GLIBC_2.36' not found (required by /lib64/libm.so.6)
assert/tst-assert-c++: /export/build/gnu/tools-build/glibc-gitlab-release/build-x86_64-linux/libc.so.6: version `GLIBC_ABI_DT_RELR' not found (required by /lib64/libm.so.6)

Pass -Wl,--disable-new-dtags to linker when building glibc tests with
--enable-hardcoded-path-in-tests.  This fixes BZ #31719.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>