Build correctly with IBT and Shadow Stack (#1289)

Add .gnu.property notes to indicate support for IBT and shadow stacks
when libsodium is built with it.  There's no stack switching code in
here, so this should not need any other codegen changes.

src/libsodium/crypto_scalarmult/curve25519/sandy2x/sandy2x.S

@@ -9,6 +9,24 @@
 #include "ladder.S"
 
 #if defined(__linux__) && defined(__ELF__)
+#if defined(__CET__)
+.section	.note.gnu.property,"a"
+.p2align 3
+.long	1f - 0f
+.long	4f - 1f
+.long	5
+0:
+.string	"GNU"
+1:
+.p2align 3
+.long	0xc0000002
+.long	3f - 2f
+2:
+.long	__CET__
+3:
+.p2align 3
+4:
+#endif
 .section .note.GNU-stack,"",%progbits
 #endif
 

src/libsodium/crypto_stream/salsa20/xmm6/salsa20_xmm6-asm.S

@@ -958,5 +958,23 @@ jmp ._bytesbetween1and255
 #endif
 
 #if defined(__linux__) && defined(__ELF__)
+#if defined(__CET__)
+.section	.note.gnu.property,"a"
+.p2align 3
+.long	1f - 0f
+.long	4f - 1f
+.long	5
+0:
+.string	"GNU"
+1:
+.p2align 3
+.long	0xc0000002
+.long	3f - 2f
+2:
+.long	__CET__
+3:
+.p2align 3
+4:
+#endif
 .section .note.GNU-stack,"",%progbits
 #endif