Generate vendor specific STUB_LIBRARIES

When generating vendor/etc/linker.config.pb, STUB_LIBRARIES was used.
Because it lists *all* libraries with stubs regardless of its
install location (vendor or system), using it for vendor
linker.config.pb doesn't make sense. Instead, use vendor-specific
STUB_LIBRARIES, which is named SOONG_STUB_VENDOR_LIBRARIES.

Bug: 313806237
Test: m (aosp_cf_x86_64_phone)
Test: $OUT/vendor/etc/linker.config.pb not listing libz.so
Change-Id: Icd0aaf92d9630c07f58c4739a9f0ac713516db43