Update BoringSSL to 21a879a78a60c8667468a9eba994c8365eaf92ea. (apple#182

)

enable @_implementationOnly for Swift 5.3 (apple#174)

Remove unsafe code from hostname verification (apple#166)

Motivation:

The hostname verification code operated on unsafe buffer pointers
without any clear justification for doing so. While it may have been
necessary to do that once, it certainly isn't necessary now.

Modifications:

Rewrite all the code to operate on Array.
Throw a more meaningful error.

Result:

Bounds checking, better safety.

Add regression limits for allocation tests. (apple#160)

Motivation:

We missed these out, which means we failed to detect the massive
allocation win in apple#159.

Modifications:

Correctly add allocation limits.

Result:

We'll know if we regress.

Add cross-compilation to symbol mangling. (apple#150)

Motivation:

BoringSSL potentially defines different symbols on different
architecture and platform combinations. This means that we've
historically missed mangling some symbol names when our script only
involved building on macOS.

We should build on more platforms to test this.

Modifications:

- Added support for cross compiling to Linux amd64, Linux aarch64, and
    Linux armv7 in symbol mangling script.
- Added support for compiling for iOS armv6 and arm64 in symbol mangling
    script.

Result:

Harder to run the mangling script, better results when you do.

Remove NIO 2.9.0 build warnings. (apple#144)

Motivation:

We were one of the few first-party users of
writeWithUnsafeMutableWritableBytes, and so we hit some build warnings.
We should use the nice interface to request the number of bytes we
actually want.

Modifications:

- Remove build warnings.
- Correctly ask to read a whole SSL record instead of a fragment of one.

Result:

Better code

Update BoringSSL to 4ca15d5dcbe6e8051a4654df7c971ea8307abfe0 (apple#142)

Use .note.GNU-stack section in ASM. (apple#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.

Don't pass read-only bytes as mutable to BoringSSL (apple#129)

Motivation:

The BoringSSL version of `BIO_new_mem_buf` takes a const buffer but
we pass a mutable one.

Modifications:

Pass a const buffer.

Result:

Unsafe instead of Unsafe+Mutable.

Fix compilation on iPad Pro 1st Gen with iPadOS13 and iOS 12 (apple#125)

Resolves an issue where the ARM architecture helpers were inadvertently excluded on 64-bit ARM platforms.