target/riscv: Disable "G" by default

target/riscv: Disable "G" by default

Because "G" virtual extension expands to "IMAFD", we cannot separately
disable extensions like "F" or "D" without disabling "G".  Because all
"IMAFD" are enabled by default, it's harmless to disable "G" by default.

Signed-off-by: Tsukasa OI <research_trasio@irq.a4lg.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <cab7205f1d7668f642fa242386543334af6bc1bd.1652583332.git.research_trasio@irq.a4lg.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>