elf: don't clobber ld.so.conf in tst-glibc-hwcaps-prepend-cache [BZ #34210]

elf: don't clobber ld.so.conf in tst-glibc-hwcaps-prepend-cache [BZ #34210]

dbe5065f2166be20e57a24f246a40d50e001a05d and ae589cb84df10825fc545a45c7007a5f79409bf1
cater for setups where ld.so.conf{,.d} is required to find runtime support
libraries, but tst-glibc-hwcaps-prepend-cache clobbers the created ld.so.conf
with its own entry.

Fix it to instead use the ld.so.conf.d created in ae589cb84df10825fc545a45c7007a5f79409bf1
to co-exist with existing entries.

Bug: https://bugs.gentoo.org/976773
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31901
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=34210
Tested-by: Andreas K. Hüttel <dilfridge@gentoo.org>
Reported-by: Eli Schwartz <eschwartz@gentoo.org>
Reviewed-by: Andreas K. Hüttel <dilfridge@gentoo.org>