target/riscv: calloc() memory per register.

target/riscv: calloc() memory per register.

This replaces a static array with 8 bytes per register. When there are
vector registers larger than 8 bytes, they would end up clobbering each
other's values. I can't believe I didn't catch this earlier.

See https://github.com/riscv/riscv-openocd/pull/658

Change-Id: I9df4eaf05617a2c8df3140fff9fe53f61ab2b261
Signed-off-by: Tim Newsome <tim@sifive.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6775
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>