[GR-43610] Separate builder and image output, show assertion status.

[graalvmbot]

• separate builder and image sections
• make clear we are talking about the image's GC
• add assertion status of builder and image to build output

example output:

mx native-image -J-da -J-esa -ea -dsa Test
...
[1/8] Initializing...                                                                                    (5.1s @ 0.23GB)
 Builder configuration:
 - Java version: 26+12, vendor version: Oracle GraalVM 26-dev+12.1
 - Graal compiler: optimization level: 2, target machine: x86-64-v3
 - C compiler: gcc (linux, x86_64, 13.3.0)
 - Assertions: diabled, system assertions: enabled
 - 1 user-specific feature(s):
   - com.oracle.svm.thirdparty.gson.GsonFeature
 Image configuration:
 - Garbage collector: Serial GC (max heap size: 80% of RAM)
 - Assertions: enabled (class-specific config may apply), system assertions: disabled
------------------------------------------------------------------------------------------------------------------------
 1 experimental option(s) unlocked:
 - '-H:PageSize' (origin(s): command line)
------------------------------------------------------------------------------------------------------------------------
Build resources:
 - 29.80GiB of memory (47.0% of system memory, capped at 32GB)
 - 22 thread(s) (100.0% of 22 available processor(s), determined at start)
[2/8] Performing analysis...