[tsan] Shrink RiscV64 48-bit LowApp region slightly to speed up TSan RestoreAddr (#72316)

The RiscV64 48-bit mappings introduced in
46cb8d9a necessitated changing
RestoreAddr to use 4-bits as the indicator. This roughly halves the
speed of RestoreAddr, because it is now brute-force testing addresses in
1TB increments, rather than 2TB increments. Crucially, this slowdown
applies to TSan on all platforms, not just RiscV64 48-bit.

This patch slightly shrinks the RiscV64 48-bit LowApp region mapping
(from 5TB to 4TB); we hope that 4TB ought to be enough for anybody,
especially since there is no ASLR applied to the binary in this region.
This allows restoring RestoreAddr to use 3-bits as the indicator again,
thus speeding up TSan on all platforms.

Co-authored-by: Thurston Dang <thurston@google.com>