[libc][arm][setjmp/longjmp] enforce minimum sizes

[nickdesaulniers]

https://github.com/ARM-software/abi-aa/blob/main/clibabi32/clibabi32.rst#setjmph

The minimum jmp_buf size is calculated as follows:
SP, LR: 2x4; reserved to setjmp implementation: 4x4; Total 3x8 bytes
General purpose register save: 8x4; Total 4x8 bytes
Floating point register save: 8x8; Total 8x8 bytes
WMMX (if present): 5x8; Total 5x8 bytes
Total: 20x8 = 160 bytes = 20 8-byte double-words.
If WMMX can be guaranteed not to be present this can be reduced to 15x8 = 120 bytes.
If floating point hardware can be guaranteed not to be present this can be further reduced to 7x8 = 56 bytes.
An implementation may define the size of a jmp_buf to be larger than the ABI-defined minimum size.

We can enforce this via static_asserts in the implementations of setjmp+longjmp. We don't plan to support WMMX.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (paternity leave) (nickdesaulniers)

https://github.com/ARM-software/abi-aa/blob/main/clibabi32/clibabi32.rst#setjmph

> The minimum jmp_buf size is calculated as follows:
SP, LR: 2x4; reserved to setjmp implementation: 4x4; Total 3x8 bytes
General purpose register save: 8x4; Total 4x8 bytes
Floating point register save: 8x8; Total 8x8 bytes
WMMX (if present): 5x8; Total 5x8 bytes
Total: 20x8 = 160 bytes = 20 8-byte double-words.
If WMMX can be guaranteed not to be present this can be reduced to 15x8 = 120 bytes.
If floating point hardware can be guaranteed not to be present this can be further reduced to 7x8 = 56 bytes.
An implementation may define the size of a jmp_buf to be larger than the ABI-defined minimum size.

We can enforce this via static_asserts in the implementations of setjmp+longjmp. We don't plan to support WMMX.