Make mjsan.h compile under strict C11 with GCC

[teerthsharma]

Summary

mjsan.h replaces mj_markStack and mj_freeStack with sanitizer-aware definitions, so it is compiled only when a sanitizer is enabled. Two constructs in those definitions are rejected by GCC in strict C11 mode, which the project selects with -std=c11 -Wpedantic -Werror.

The attribute follows the declarator in a function definition, which GCC rejects with "attributes should be specified before the declarator in a function definition". The barrier uses the bare keyword asm, which is not reserved in strict C11; GCC requires __asm__ there.

Both move to the accepted spelling. Generated code is unchanged: always_inline still applies, and __asm__ volatile is the same barrier. Six lines, one file. There is no API, semantics, or non-sanitizer build change.

Reproduction

On f9a00bd5, Ubuntu 26.04, GCC 15.2.0:

gcc -std=c11 -Wpedantic -Werror -fsanitize=address \
    -Iinclude -Isrc -c src/engine/engine_collision_box.c -o /dev/null

	exit	diagnostics
before	1	mjsan.h:58 and mjsan.h:65, attributes should be specified before the declarator
after	0	none

Any translation unit reaching mj_markStack or mj_freeStack fails identically, so an ASan build of the library does not get past its first engine source file.

Coverage

Measured after the change, same host and flags:

configuration	exit
GCC 15.2.0, -fsanitize=address	0
clang 21.1.8, -fsanitize=address	0
GCC 15.2.0, no sanitizer	0

The third row is the control: without a sanitizer the header takes its other branch, so it confirms the ordinary build path is untouched.

Limits

Verified on Linux x86-64 with GCC 15.2.0 and clang 21.1.8 only. Older GCC accepts the trailing-attribute form, so this is a portability fix rather than a correctness fix, and it is invisible unless a sanitizer is enabled. Windows and macOS were not exercised, though neither construct is compiler-specific in the direction that would newly break them.