Fix stub of sockets for EMSCRIPTEN and XROS

Summary:
The current implementation of function stubs in `SocketFileDescriptorMap.cpp` generates the following build errors:
```
stderr: xplat/folly/net/detail/SocketFileDescriptorMap.cpp:171:3: error: 'socketToFd' has a non-throwing exception specification but can still throw [-Werror,-Wexceptions]
  throw std::logic_error("Not implemented!");
  ^
xplat/folly/net/detail/SocketFileDescriptorMap.cpp:170:30: note: function declared non-throwing here
int SocketFileDescriptorMap::socketToFd(void* sock) noexcept {
```
because the methods are stubbed out to throw and exception even though they are marked as `noexcept`.

To fix the warning the subbing implementation is changed to call `std::terminate()` instead of throwing an exception. According to the language specification (https://en.cppreference.com/w/cpp/language/noexcept_spec) this should not result in any change in run-time behavior, since throwing and exception in a method marked as `noexcept` is effectively a call to `std::terminate()`.

Differential Revision: D29687674

fbshipit-source-id: 77405d8a31e45c8836e8746c9b25e12ef06335c4

Add computeChainCapacity function to IOBuf

Summary:
This diff adds a computeChainCapacity method to IOBuf.

This method will loop through each chain of an IOBuf and return the total capacity of all IOBuf chains. This method is used very similarly to computeChainDataLength(), except rather than returning total data length, it returns total capacity.

Reviewed By: yfeldblum

Differential Revision: D28875393

fbshipit-source-id: 7e5f4a93916fa6c30eef8f8ee8c346a4537302af

Add rsa_pss signature schemes to SSLCommonOptions

Summary: Required to support RSA certificates in TLS 1.3.

Reviewed By: knekritz

Differential Revision: D28606758

fbshipit-source-id: 0b75eccf207c1327e590675137ce3289956d8771

Add rsa_pss signature schemes to SSLCommonOptions

Summary: Required to support RSA certificates in TLS 1.3.

Reviewed By: knekritz

Differential Revision: D28606758

fbshipit-source-id: 0b75eccf207c1327e590675137ce3289956d8771

Add rsa_pss signature schemes to SSLCommonOptions

Summary: Required to support RSA certificates in TLS 1.3.

Reviewed By: knekritz

Differential Revision: D28606758

fbshipit-source-id: 0b75eccf207c1327e590675137ce3289956d8771

hazptr: Improve documentation

Summary: Improve documentation and make the description of synchronous reclamation  consistent with the current implementation.

Reviewed By: yfeldblum

Differential Revision: D28575528

fbshipit-source-id: e8f37e02d18b12a0e653264c128492a98cba6a1d

fix exception_ptr_get_type for libc++ < v10.0.0-rc2

Summary:
The function `folly::exception_ptr_get_type` under 32-bit arm-ehabi libc++ >= v5.0.1 and < v10.0.0-rc2, is broken because the `__cxa_exception` layout is changed in an incompatible way, with an extra 4 bytes padding incorrectly preceding the unwind field. Shift the `__cxa_exception` pointer before accessing fields through it.

Major thanks to Crystal Jin <[email protected]> for help debugging the problem and testing the fix.

Differential Revision: D28423734

fbshipit-source-id: fdf67fa5c1056df921fad23f86ae965761603738

Back out "exception_wrapper thrown variant via abi/runtime"

Summary: D26331579 (facebook@aa7f74a) was the offending diff that caused an insta crash on Messenger Android and Instagram Android. Backing out for now.

Reviewed By: JunqiWang

Differential Revision: D28308563

fbshipit-source-id: 87a547d9ba7cb3b33a4ddee9e273943f8379d990

revise exceptionStr

Summary: Revise all overloads of `folly::exceptionStr` in terms of `type_info_of`, `exception_ptr_get_type`, and `exception_ptr_get_object`. No longer rely on `catch_exception` and no longer have inline preprocessor conditionals.

Reviewed By: Orvid, luciang

Differential Revision: D26333081

fbshipit-source-id: 318ce83b9f15a12d5a33f528134e6fb38bb78a62

use only public gmock interface in transport/socket lifecycle observer

Summary: Internals are not portable between versions or across platforms.

Reviewed By: Orvid

Differential Revision: D27990286

fbshipit-source-id: b70d8d3f67a1e5262efa842f70817e3ae3972339