Tweak compiler version cutoff for no_sanitize("alignment") support.

Buildfarm results show that gcc up through 7.x produces annoying
warnings for this construct (and, presumably, wouldn't do the right
thing anyway).  clang seems okay with the cutoff we have, though.

Discussion: https://postgr.es/m/CAPpHfdsne3%3DT%3DfMNU45PtxdhSL_J2PjLTeS8rwKnJzUR4YNd4w%40mail.gmail.com
Discussion: https://postgr.es/m/475514.1612745257%40sss.pgh.pa.us

src/include/c.h

@@ -138,7 +138,7 @@
  * Testing can be done with "-fsanitize=alignment -fsanitize-trap=alignment"
  * on clang, or "-fsanitize=alignment -fno-sanitize-recover=alignment" on gcc.
  */
-#if __clang_major__ >= 7 || __GNUC__ >= 5
+#if __clang_major__ >= 7 || __GNUC__ >= 8
 #define pg_attribute_no_sanitize_alignment() __attribute__((no_sanitize("alignment")))
 #else
 #define pg_attribute_no_sanitize_alignment()