target/riscv: report helpfull location during register decode

[en-sc]

LOG_TARGET_DEBUG() reports file, line and function name at the call
site. This information is not helpfull if it always points to the same
location inside log_debug_reg().

Additionally, avoid using VLA in log_debug_reg(), since OpenOCD style guide(doc/manual/style.txt) prohibits it:

• use malloc() to create dynamic arrays. Do @b not use @c alloca
  or variable length arrays on the stack. non-MMU hosts(uClinux) and
  pthreads require modest and predictable stack usage.

[timsifive]

Looks good. Can you share a sample output line emitted by the new code?

[en-sc]

Prior to the change:

Debug: 82 2 riscv-013.c:307 log_debug_reg(): [riscv.cpu] dtmcs=0x61 { version=1.0, dmistat=0, idle=0, dmireset=0, dtmhardreset=0, errinfo=not implemented, abits=6, }

After:

Debug: 82 3 riscv-013.c:1870 examine(): [riscv.cpu] dtmcs=0x61 { version=1.0, dmistat=0, idle=0, dmireset=0, dtmhardreset=0, errinfo=not implemented, abits=6, }

This PR does not update debug_reg_printer.[ch], so the actual decoding string is flawed. Please, see this comment.