python3: Fix readelf program name not replaced in _sysconfigdata.py

python3: Fix readelf program name not replaced in _sysconfigdata.py

The Makefile lines to add READELF to TARGET_CONFIGURE_OPTS was removed
in 4e05541782edeb06b51d691dadf52648df24c940.

Without setting READELF, configure finds the symlink to
$(TARGET_CROSS)readelf (e.g. arm-openwrt-linux-readelf) instead of
$(TARGET_CROSS)readelf (e.g. arm-openwrt-linux-muslgnueabi-readelf).

This leads to the symlink name being saved to _sysconfigdata.py, and so
the readelf name is not replaced correctly (in
Py3Package/python3-base/install).

This restores the removed Makefile lines.

Fixes: 4e05541782ed ("python3: bump to version 3.10.0")
Signed-off-by: Jeffery To <jeffery.to@gmail.com>
(cherry picked from commit e1a95786358c64483fc16d52eac6746267f0abec)