[ASan][RISCV] Fix RISC-V memory mapping

Fixes the ASan RISC-V memory mapping (originally introduced by D87580 and
D87581). This should be an improvement both in terms of first principles
soundness and observed test failures --- test failures would occur
non-deterministically depending on the ASLR random offset.

On RISC-V Linux (64-bit), `TASK_UNMAPPED_BASE` is currently defined as
`PAGE_ALIGN(TASK_SIZE / 3)`. The non-power-of-two divisor makes the result
be the not very round number 0x1555556000. That address had to be further
rounded to ensure page alignment after the shadow scale shifting is applied.
Still, that value explains why the mapping table may look less regular than
expected.

Further cleanups:
- Moved the mapping table comment, to ensure that the two Linux/AArch64
tables stayed together;
- Removed mention of Sv48. Neither the original mapping nor this one are
compatible with an actual Linux Sv48 address space (mainline Linux still
operates Sv48 in Sv39 mode). A future patch can improve this;
- Removed the additional comments, for consistency.

Differential Revision: https://reviews.llvm.org/D97646

lib/Transforms/Instrumentation/AddressSanitizer.cpp

@@ -105,7 +105,7 @@ static const uint64_t kSystemZ_ShadowOffset64 = 1ULL << 52;
 static const uint64_t kMIPS32_ShadowOffset32 = 0x0aaa0000;
 static const uint64_t kMIPS64_ShadowOffset64 = 1ULL << 37;
 static const uint64_t kAArch64_ShadowOffset64 = 1ULL << 36;
-static const uint64_t kRISCV64_ShadowOffset64 = 0x20000000;
+static const uint64_t kRISCV64_ShadowOffset64 = 0xd55550000;
 static const uint64_t kFreeBSD_ShadowOffset32 = 1ULL << 30;
 static const uint64_t kFreeBSD_ShadowOffset64 = 1ULL << 46;
 static const uint64_t kNetBSD_ShadowOffset32 = 1ULL << 30;