[testsuite] Avoid duplicate test names in sizeless tests

[testsuite] Avoid duplicate test names in sizeless tests

Jeff pointed out that using:

   N:  ... /* { dg-error {...} } */
 N+1:      /* { dg-error {...} "" { target *-*-* } .-1 } */

led to two identical test names for line N.  Fixed by adding
a proper test name instead of "".

2020-03-16  Richard Sandiford  <richard.sandiford@arm.com>

gcc/testsuite/
	* gcc.target/aarch64/sve/acle/general-c/sizeless-1.c: Add a test
	name to .-1 dg-error tests.
	* gcc.target/aarch64/sve/acle/general-c/sizeless-2.c: Likewise.