Fix the osx arm64 build.

[vargaz]

No description provided.

[directhex]

I'm seeing

/Users/directhex/Projects/runtime/src/mono/mono/utils/mono-context.c
  /Users/directhex/Projects/runtime/src/mono/mono/utils/mono-context.c:560:2: error: implicit declaration of function 'UCONTEXT_REG_SET_SP' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
          UCONTEXT_REG_SET_SP (sigctx, mctx->regs [ARMREG_SP]);
          ^
  1 error generated.

Does your change also need a definition for UCONTEXT_REG_SET_SP, as well as UCONTEXT_REG_SET_PC?

[directhex]

Do we have to wait on (broken) CI here? Can we even validate this in CI?

[CoffeeFlux]

In theory the install script issue should be fixed now, so restarting the lanes.

[steveisok]

I incorporated it into #47864 and yes, it works. Presumably we could close this PR.