target/riscv: misa to ISA string conversion fix

target/riscv: misa to ISA string conversion fix

Some bits in RISC-V `misa' CSR should not be reflected in the ISA
string.  For instance, `S' and `U' (represents existence of supervisor
and user mode, respectively) in `misa' CSR must not be copied since
neither `S' nor `U' are valid single-letter extensions.

This commit also removes all reserved/dropped single-letter "extensions"
from the list.

-   "B": Not going to be a single-letter extension (misa.B is reserved).
-   "J": Not going to be a single-letter extension (misa.J is reserved).
-   "K": Not going to be a single-letter extension (misa.K is reserved).
-   "L": Dropped.
-   "N": Dropped.
-   "T": Dropped.

It also clarifies that the variable `riscv_single_letter_exts' is a
single-letter extension order list.

Signed-off-by: Tsukasa OI <research_trasio@irq.a4lg.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <4a4c11213a161a7eedabe46abe58b351bb0e2ef2.1648473008.git.research_trasio@irq.a4lg.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>