So I'm not super familiar with those configure macros, but I noticed this in configure.ac"

# Check if asm support armv8.2-a+sve
AC_CACHE_CHECK(for SVE support in assembler, libc_cv_asm_sve, [dnl
cat > conftest.s <<\EOF
        ptrue p0.b
EOF
if AC_TRY_COMMAND(${CC-cc} -c -march=armv8.2-a+sve conftest.s 1>&AS_MESSAGE_LOG_FD); then
  libc_cv_aarch64_sve_asm=yes
else
  libc_cv_aarch64_sve_asm=no
fi
rm -f conftest*])
if test $libc_cv_aarch64_sve_asm = yes; then
  AC_DEFINE(HAVE_AARCH64_SVE_ASM)
fi

Note the use of "libc_cv_asm_sve" vs. "libc_cv_aarch64_sve_asm"

This doesn't match other tests in the same file and might explain why the result isn't displayed properly when running configure...