test-lib: add prerequisite for 64-bit platforms

Allow tests that assume a 64-bit `size_t` to be skipped in 32-bit
platforms and regardless of the size of `long`.

This imitates the `LONG_IS_64BIT` prerequisite.

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -1686,6 +1686,10 @@ build_option () {
 	sed -ne "s/^$1: //p"
 }
 
+test_lazy_prereq SIZE_T_IS_64BIT '
+	test 8 -eq "$(build_option sizeof-size_t)"
+'
+
 test_lazy_prereq LONG_IS_64BIT '
 	test 8 -le "$(build_option sizeof-long)"
 '