make.defaults: negative incrementals in USE_EXPAND (530222)

Previously, USE_EXPAND variable settings in profile make.defaults only
supported positive incremental settings. This patch adds support for
negative settings like PYTHON_TARGETS="-python3_3", which brings
behavior into alignment with PMS.

Notably, this patch does not change behavior for settings in make.conf.
In make.conf, settings to USE_EXPAND variables remain entirely
non-incremental. PMS does not govern make.conf behavior.

X-Gentoo-Bug: 530222
X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=530222
Acked-by: Brian Dolbec <dolsen@gentoo.org>