RISC-V: announce real and available memory at boot

[mhorne]

Summary

Most architectures print their total (real) and available memory during
boot. Properly initialize the realmem global and print these messages.
Also print the physical memory chunks (behind a bootverbose flag).

Test Plan

Sample output:

CPU(0): Unknown Implementer Unknown Processor
real memory  = 2146435072 (2047 MB)
Physical memory chunk(s):
0x00000000809fd000 - 0x00000000809fffff, 12288 bytes (3 pages)
0x0000000080b00000 - 0x00000000fcab2fff, 2080059392 bytes (507827 pages)
avail memory = 2070695936 (1974 MB)

[markj]

Approved.