The sets host Python's prefix and exec-prefix to the same value
($(STAGING_DIR)/host for packages, as currently defined in
include/host-build.mk), which avoids the case where pyconfig.h is
not in the same location as Python's other header files (see
https://bugs.python.org/issue896330).

This also removes some cross compilation workarounds
(_python_sysroot/prefix/exec_prefix, disabling byte-compilation,
__PYVENV_LAUNCHER__) that are not necessary when compiling packages
for host.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>