Add output size parameter to signature functions

The functions mbedtls_pk_sign(), mbedtls_pk_sign_restartable(),
mbedtls_ecdsa_write_signature() and mbedtls_ecdsa_write_signature_restartable()
now take an extra parameter indicating the size of the output buffer for the
signature.

No change to RSA because for RSA, the output size is trivial to calculate.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/programs/test/benchmark.c b/programs/test/benchmark.c
index 6a53647..6f730bc 100644
--- a/programs/test/benchmark.c
+++ b/programs/test/benchmark.c
@@ -1088,7 +1088,7 @@
                                               curve_info->name );
             TIME_PUBLIC( title, "sign",
                     ret = mbedtls_ecdsa_write_signature( &ecdsa, MBEDTLS_MD_SHA256, buf, curve_info->bit_size,
-                                                tmp, &sig_len, myrand, NULL ) );
+                                                tmp, sizeof( tmp ), &sig_len, myrand, NULL ) );
 
             mbedtls_ecdsa_free( &ecdsa );
         }
@@ -1104,7 +1104,7 @@
 
             if( mbedtls_ecdsa_genkey( &ecdsa, curve_info->grp_id, myrand, NULL ) != 0 ||
                 mbedtls_ecdsa_write_signature( &ecdsa, MBEDTLS_MD_SHA256, buf, curve_info->bit_size,
-                                               tmp, &sig_len, myrand, NULL ) != 0 )
+                                               tmp, sizeof( tmp ), &sig_len, myrand, NULL ) != 0 )
             {
                 mbedtls_exit( 1 );
             }