Improve sorry message for -fzero-call-used-regs

Improve sorry message for -fzero-call-used-regs

When the -fzero-call-used-regs command line option is used with an
unsupported value, indicate that it's a value problem instead of an
option problem.

Without the patch, the error is:
In file included from gcc/testsuite/c-c++-common/zero-scratch-regs-8.c:5:
gcc/testsuite/c-c++-common/zero-scratch-regs-1.c: In function 'foo':
gcc/testsuite/c-c++-common/zero-scratch-regs-1.c:10:1: sorry, unimplemented: '-fzero-call-used-regs' not supported on this target
   10 | }
      | ^

With the patch, the error would be like this:
 In file included from gcc/testsuite/c-c++-common/zero-scratch-regs-8.c:5:
gcc/testsuite/c-c++-common/zero-scratch-regs-1.c: In function 'foo':
gcc/testsuite/c-c++-common/zero-scratch-regs-1.c:10:1: sorry, unimplemented: argument 'all-arg' is not supported for '-fzero-call-used-regs' on this target
   10 | }
      | ^

2022-09-19  Torbjörn SVENSSON  <torbjorn.svensson@foss.st.com>

gcc/ChangeLog:

	* targhooks.cc (default_zero_call_used_regs): Improve sorry
	message.

Signed-off-by: Torbjörn SVENSSON  <torbjorn.svensson@foss.st.com>