[PATCH][lld] ELF/AArch64: Add support for checking for ABS32 overflow

[Tim Northover]

> We have isInt and isIntN in MathExtras.h for this.

I think this is slightly different, permitting both signed and
unsigned ranges: 16-bits would allow from -32767 to 65535 for example.
Could still do "isIntN || isUIntN", which might well be cleaner.

Tim.