rust: Allow rust_bindgen to use cc_defaults.

rust_bindgen modules can't inherit properties in cc_defaults that would
be useful for generating bindings (such as cflags). This CL moves these
common properties out into a new struct in cc and adds that struct to
cc_default.

Additionally, Cppflags is added to rust_bindgen to make sure that these
get picked up as well from cc_defaults.

Bug: 163598610
Test: rust_bindgen module uses cflags in cc_defaults.
Test: New Soong test passes
Change-Id: I702442a355244dc01954083f98a2eebbcea12e47