Update cortex-m requirement from 0.6.0 to 0.7.2

[dependabot-preview]

Updates the requirements on cortex-m to permit the latest version.

Release notes

Sourced from cortex-m's releases.

v0.7.2

Fixed

• Fixed a bug where calling asm::delay() with an argument of 0 or 1 would underflow, leading to a very long delay.

Changelog

Sourced from cortex-m's changelog.

[v0.7.2] - 2021-03-07

Fixed

• Fixed a bug where calling asm::delay() with an argument of 0 or 1 would underflow, leading to a very long delay.

[v0.7.1] - 2021-01-25

Added

• New assembly methods asm::semihosting_syscall, asm::bootstrap, and asm::bootload.

Deprecated

• msp::write has been deprecated in favor of asm::bootstrap. It was not possible to use msp::write without causing Undefined Behavior, so all existing users are encouraged to migrate.

Fixed

• Fixed a bug in asm::delay which could lead to incorrect codegen and infinite loops.
• Improved timing guarantees of asm::delay on multiple-issue CPU cores.
• Additional compiler fences added to inline assembly where necessary.
• Fixed DWARF debug information in pre-built assembly binaries.

[v0.7.0] - 2020-11-09

Added

• New InterruptNumber trait is now required on interrupt arguments to the various NVIC functions, replacing the previous use of Nr from bare-metal. For backwards compatibility, InterruptNumber is implemented for types which are Nr + Copy, but this will be removed in a future version.
• Associated const PTR is introduced to Core Peripherals to eventually replace the existing ptr() API.
• A delay driver based on SysTick.
• You can now use LTO to inline assembly calls, even on stable Rust. See the asm/lib.rs documentation for more details.
• Initial ARMv8-M MPU support
• ICTR and ACTLR registers added
• Support for the Security Attribution Unit on ARMv8-M

Changed

• Previously, asm calls without the inline-asm feature enabled used pre-built objects which were built by a GCC compiler, while inline-asm enabled the use of llvm_asm! calls. The asm system has been replaced with a new

... (truncated)

Commits

• 653d218 Merge #335
• 098d193 Prepare for v0.7.2 release of cortex-m
• e8179ec Merge #334
• e802d0e Remove an extra function call in primask::read
• 7481f09 Merge #330
• 3a5162b Fix rust-lld: error: undefined symbol: __syscall
• 2281fd6 Merge #328
• fdc3ab0 Update binaries
• aa37934 xtask assemble
• 7b66016 Extra comment
• Additional commits viewable in compare view

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