Use .note.GNU-stack section in ASM. (#137)

Motivation:

Well-formed Linux assembly code declares a .note.GNU-stack section in
order to help out the linker. When absent, the linker makes some
suboptimal linking decisions that can cause pain in running programs.

Modifications:

- Amend the vendoring script to add the .note.GNU-stack section to all
    assembly files on Linux.
- Updated BoringSSL

Results:

Better linker behaviour.

Author: Lukasa

Package.swift

@@ -22,7 +22,7 @@ import PackageDescription
 // Sources/CNIOBoringSSL directory. The source repository is at
 // https://boringssl.googlesource.com/boringssl.
 //
-// BoringSSL Commit: 9806ae005b3085683a2fcf2953e4170893e0973d
+// BoringSSL Commit: 6e7255c17e1a7348a2377fbc804441dd284806e2
 
 let package = Package(
     name: "swift-nio-ssl",

Sources/CNIOBoringSSL/crypto/asn1/tasn_enc.c

@@ -192,7 +192,7 @@ int ASN1_item_ex_i2d(ASN1_VALUE **pval, unsigned char **out,
         /* Use indefinite length constructed if requested */
         if (aclass & ASN1_TFLG_NDEF)
             ndef = 2;
-        /* fall through */
+        OPENSSL_FALLTHROUGH;
 
     case ASN1_ITYPE_SEQUENCE:
         i = asn1_enc_restore(&seqcontlen, out, pval, it);

Sources/CNIOBoringSSL/crypto/chacha/chacha-armv4.ios.arm.S

@@ -1499,3 +1499,7 @@ OPENSSL_armcap_P:
 #endif
 #endif  // !OPENSSL_NO_ASM
 #endif  // defined(__arm__) && defined(__APPLE__)
+#if defined(__linux__) && defined(__ELF__)
+.section .note.GNU-stack,"",%progbits
+#endif
+

Sources/CNIOBoringSSL/crypto/chacha/chacha-armv4.linux.arm.S

@@ -1493,3 +1493,7 @@ ChaCha20_neon:
 #endif
 #endif  // !OPENSSL_NO_ASM
 #endif  // defined(__arm__) && defined(__linux__)
+#if defined(__linux__) && defined(__ELF__)
+.section .note.GNU-stack,"",%progbits
+#endif
+

Sources/CNIOBoringSSL/crypto/chacha/chacha-armv8.ios.aarch64.S

@@ -1983,3 +1983,7 @@ Ldone_512_neon:
 
 #endif  // !OPENSSL_NO_ASM
 #endif  // defined(__aarch64__) && defined(__APPLE__)
+#if defined(__linux__) && defined(__ELF__)
+.section .note.GNU-stack,"",%progbits
+#endif
+

Sources/CNIOBoringSSL/crypto/chacha/chacha-armv8.linux.aarch64.S

@@ -1985,3 +1985,7 @@ ChaCha20_512_neon:
 #endif
 #endif  // !OPENSSL_NO_ASM
 #endif  // defined(__aarch64__) && defined(__linux__)
+#if defined(__linux__) && defined(__ELF__)
+.section .note.GNU-stack,"",%progbits
+#endif
+

Sources/CNIOBoringSSL/crypto/chacha/chacha-x86.linux.x86.S

@@ -975,3 +975,7 @@ ChaCha20_ssse3:
 .byte	114,103,62,0
 #endif
 #endif  // defined(__i386__) && defined(__linux__)
+#if defined(__linux__) && defined(__ELF__)
+.section .note.GNU-stack,"",%progbits
+#endif
+

Sources/CNIOBoringSSL/crypto/chacha/chacha-x86_64.linux.x86_64.S

@@ -1633,3 +1633,7 @@ ChaCha20_8x:
 .size	ChaCha20_8x,.-ChaCha20_8x
 #endif
 #endif  // defined(__x86_64__) && defined(__linux__)
+#if defined(__linux__) && defined(__ELF__)
+.section .note.GNU-stack,"",%progbits
+#endif
+

Sources/CNIOBoringSSL/crypto/chacha/chacha-x86_64.mac.x86_64.S

@@ -1626,3 +1626,7 @@ L$8x_epilogue:
 
 #endif
 #endif  // defined(__x86_64__) && defined(__APPLE__)
+#if defined(__linux__) && defined(__ELF__)
+.section .note.GNU-stack,"",%progbits
+#endif
+

Sources/CNIOBoringSSL/crypto/cipher_extra/aes128gcmsiv-x86_64.linux.x86_64.S

@@ -3079,3 +3079,7 @@ aes256gcmsiv_kdf:
 .size	aes256gcmsiv_kdf, .-aes256gcmsiv_kdf
 #endif
 #endif  // defined(__x86_64__) && defined(__linux__)
+#if defined(__linux__) && defined(__ELF__)
+.section .note.GNU-stack,"",%progbits
+#endif
+