Use _Static_assert in refcount_c11.c to support old compilers that don't support the macro static_assert

[andrewhop]

Description of changes:

Some other old compilers don't support the convince macro static_assert but do support the C11 keyword _Static_assert even though both should be in assert.h.

Testing:

Built all old platforms internally.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.33%. Comparing base (7d3101f) to head (6cc8758).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1789      +/-   ##
==========================================
- Coverage   78.33%   78.33%   -0.01%     
==========================================
  Files         580      580              
  Lines       97256    97255       -1     
  Branches    13945    13946       +1     
==========================================
- Hits        76187    76186       -1     
- Misses      20445    20448       +3     
+ Partials      624      621       -3     

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[justsmth]

We use OPENSSL_STATIC_ASSERT other places for this same purpose. It's defined here:

aws-lc/include/openssl/type_check.h

Lines 67 to 98 in 7d3101f

	// Previously we defined |OPENSSL_STATIC_ASSERT| to use one of two keywords:
	// |Static_assert| or |static_assert|. The latter was used if we were compiling
	// a C++ translation unit or on Windows (excluding when using a Clang compiler).
	// The former was used in other cases. However, these two keywords are not
	// defined before C11. So, we can't rely on these when we want to be C99
	// compliant. If we at some point decide that we want to only be compliant with
	// C11 (and up), we can reintroduce these keywords. Instead, use a method that
	// is guaranteed to be C99 compliant and still give us an equivalent static
	// assert mechanism.
	//
	// The solution below defines a struct type containing a bit field.
	// The name of that type is |static_assertion_msg|. |msg| is a concatenation of
	// a user-chosen error (which should be chosen with respect to actual assertion)
	// and the line the assertion is defined. This should ensure name uniqueness.
	// The width of the bit field is set to 1 or -1, depending on the evaluation of
	// the boolean expression |cond|. If the condition is false, the width requested
	// is -1, which is illegal and would cause the compiler to throw an error.
	//
	// An example of an error thrown during compilation:
	// ```
	// error: negative width in bit-field
	// 'static_assertion_at_line_913_error_is_AEAD_state_is_too_small'
	// ```
	#define AWSLC_CONCAT(left, right) left##right
	#define AWSLC_STATIC_ASSERT_DEFINE(cond, msg) typedef struct { \
	unsigned int AWSLC_CONCAT(static_assertion_, msg) : (cond) ? 1 : - 1; \
	} AWSLC_CONCAT(static_assertion_, msg) OPENSSL_UNUSED;
	#define AWSLC_STATIC_ASSERT_ADD_LINE0(cond, suffix) AWSLC_STATIC_ASSERT_DEFINE(cond, AWSLC_CONCAT(at_line_, suffix))
	#define AWSLC_STATIC_ASSERT_ADD_LINE1(cond, line, suffix) AWSLC_STATIC_ASSERT_ADD_LINE0(cond, AWSLC_CONCAT(line, suffix))
	#define AWSLC_STATIC_ASSERT_ADD_LINE2(cond, suffix) AWSLC_STATIC_ASSERT_ADD_LINE1(cond, __LINE__, suffix)
	#define AWSLC_STATIC_ASSERT_ADD_ERROR(cond, suffix) AWSLC_STATIC_ASSERT_ADD_LINE2(cond, AWSLC_CONCAT(_error_is_, suffix))
	#define OPENSSL_STATIC_ASSERT(cond, error) AWSLC_STATIC_ASSERT_ADD_ERROR(cond, error)