Create portable box helper and wrap the thread-local alloc context in a struct

[jkoritzinsky]

Create a portable box helper and remove the assembly-based one. The codegen is close, but we don't need to be exact as most box operations are inlined anyway

Follow-ups to #103055

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[jkotas]

LGTM otherwise. Thank you!

[jkotas]

Arm32 is failing with asserts like: Assert failure(PID 20 [0x00000014], Thread: 38 [0x0026]): !CREATE_CHECK_STRING((((size_t)this) & 0x7) == (size_t)(pMT->IsValueType()?4:0))

Looks like a problem caused by FEATURE_64BIT_ALIGNMENT.

[jkotas]

cc @elinor-fung for the cDAC change

[jkoritzinsky]

/ba-g build failure is linked to a known test failure