RFR(S) 8233093: Move CDS heap oopmaps into new MetaspaceShared::bm region

[Calvin Cheung]

RFE: https://bugs.openjdk.java.net/browse/JDK-8233093

webrev: http://cr.openjdk.java.net/~ccheung/jdk15/8233093/webrev.00/

The MetaspaceShared::bm region was introduced via JDK-8231610 (Relocate 
the CDS archive if it cannot be mapped to the requested address) and 
only used during archive loading. The region will be unmapped afterwards 
to save memory footprint. Since the heap oopmaps are also only used 
during archive loading, it makes sense for them to reside in the "bm" 
region as well.

Before:
[0.753s][debug][cds] ro space: 7552120 [ 59.7% of total] out of 7553024 
bytes [100.0% used] at 0x00000008003e1000
[0.753s][debug][cds] bm space: 184320 [ 1.5% of total] out of 184320 
bytes [100.0% used] at 0x0000000000000000

After:
[0.766s][debug][cds] ro space: 7525856 [ 59.5% of total] out of 7528448 
bytes [100.0% used] at 0x00000008003e1000
[0.766s][debug][cds] bm space: 208896 [ 1.7% of total] out of 208896 
bytes [100.0% used] at 0x0000000000000000

Passed tiers 1 - 4 testing.

thanks,

Calvin