The ldrexd/strexd instructions are not supported on M-class chips, see for example https://developer.arm.com/documentation/dui0489/e/arm-and-thumb-instructions/memory-access-instructions/ldrex-and-strex which says:
All these 32-bit Thumb instructions are available in ARMv6T2 and above, except that LDREXD and STREXD are not available in the ARMv7-M architecture.
Looking at the ARMv8-M architecture, it appears that these instructions aren't supported either. The Architecture Reference Manual lists ldrex/strex but not ldrexd/strexd: https://developer.arm.com/documentation/ddi0553/bn/
Compiler Explorer example on LLVM 11.0.0, which incorrectly emits ldrexd/strexd instructions: https://llvm.godbolt.org/z/5qqPnE