Create linker_config_srcs for autogenerated vendor partition

This will be used to create /vendor/etc/linker.config.pb. Since
`PRODUCT_VENDOR_LINKER_CONFIG_FRAGMENTS` are relative to workspace top,
a filegroup soong module will be dynamically created.

Test: checked that /etc/linker.config.pb is present in autogenerated
vendor.img

Bug: 375686533

Change-Id: I73269e403f95c1b7f4b02a26c0d7a0f564876950