Use LDREX/STREX on ARM cortex for mutex

[hathach]

Is your feature request related to a problem? Please describe.
LDREX/STREX instructions are available on ARM cortex M3/M4/M7 which can be used for mutex on multiple cores MCUs

Reference

• clean up pwn and analog adafruit/Adafruit_nRF52_Arduino#562 (comment)
• https://devblogs.microsoft.com/oldnewthing/20180329-00/?p=98375
• https://community.arm.com/developer/ip-products/processors/f/cortex-m-forum/10361/ldrex-strex-on-the-m3-m4-m7
• https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html#g_t_005f_005fatomic-Builtins
• https://en.cppreference.com/w/c/atomic