[compiler-rt][ARM] Double-precision FP support functions (#179920)

This commit adds C helper functions `dnan2`, `dnorm2` and `dunder` for
handling the less critical edge cases of double-precision arithmetic,
similar to `fnan2`, `fnorm2` and `funder` that were added in commit
f7e652127772e93.

It also adds a header file that defines some register aliases for
handling double-precision numbers in AArch32 software floating point in
an endianness-independent way, by providing aliases `xh` and `xl` for
the high and low words of the first double-precision function argument,
regardless of which of them is in r0 and which in r1, and similarly `yh`
and `yl` for the second argument in r2/r3.

NOKEYCHECK=True
GitOrigin-RevId: 80831832e03f8184fb24ffd08ccba59d4e57ec0e