guile: Change return value of gdbscm_with_guile for const char *

guile: Change return value of gdbscm_with_guile for const char *

The documentation of gdbscm_with_guile says that it returns a statically
allocated string (IOW, a const char *).  We can reflect that in its
return value type, and get rid of C++ build errors.

Initially fixes:

/home/simark/src/binutils-gdb/gdb/guile/scm-disasm.c: In function ‘void* gdbscm_disasm_read_memory_worker(void*)’:
/home/simark/src/binutils-gdb/gdb/guile/scm-disasm.c:93:12: error: invalid conversion from ‘const void*’ to ‘void*’ [-fpermissive]
     return "seek error";

gdb/ChangeLog:

	* guile/guile-internal.h (gdbscm_with_guile): Change return
	types to const char *.
	* guile/scm-safe-call.c (gdbscm_with_guile): Likewise.
	(struct c_data) <func>: Likewise.
	(struct c_data) <result>: Change type to const char *.
	(scscm_eval_scheme_string): Change return type to
	const char *.
	(scscm_source_scheme_script): Likewise.
	(gdbscm_safe_eval_string): Change type of result variable to
	const char * and remove cast.
	(gdbscm_safe_source_script): Likewise.
	* guile/scm-disasm.c (gdbscm_disasm_read_memory_worker):
	Change return type to const char *.
	(gdbscm_disasm_read_memory): Change type of status to
	const char *.