pg_attribute_no_sanitize_alignment() macro

Modern gcc and clang compilers offer alignment sanitizers, which help to detect
pointer misalignment.  However, our codebase already contains x86-specific
crc32 computation code, which uses unalignment access.  Thankfully, those
compilers also support the attribute, which disables alignment sanitizers at
the function level.  This commit adds pg_attribute_no_sanitize_alignment(),
which wraps this attribute, and applies it to pg_comp_crc32c_sse42() function.

Back-patch of commits 993bdb9f9 and ad2ad698a, to enable doing
alignment testing in all supported branches.

Discussion: https://postgr.es/m/CAPpHfdsne3%3DT%3DfMNU45PtxdhSL_J2PjLTeS8rwKnJzUR4YNd4w%40mail.gmail.com
Discussion: https://postgr.es/m/475514.1612745257%40sss.pgh.pa.us
Author: Alexander Korotkov, revised by Tom Lane
Reviewed-by: Tom Lane

src/include/c.h

@@ -139,6 +139,18 @@
 #define pg_attribute_unused()
 #endif
 
+/*
+ * Place this macro before functions that should be allowed to make misaligned
+ * accesses.  Think twice before using it on non-x86-specific code!
+ * 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__ >= 8
+#define pg_attribute_no_sanitize_alignment() __attribute__((no_sanitize("alignment")))
+#else
+#define pg_attribute_no_sanitize_alignment()
+#endif
+
 /*
  * Append PG_USED_FOR_ASSERTS_ONLY to definitions of variables that are only
  * used in assert-enabled builds, to avoid compiler warnings about unused

src/port/pg_crc32c_sse42.c

@@ -18,6 +18,7 @@
 
 #include <nmmintrin.h>
 
+pg_attribute_no_sanitize_alignment()
 pg_crc32c
 pg_comp_crc32c_sse42(pg_crc32c crc, const void *data, size_t len)
 {