configure: add --enable-sysroot

The `--enable-sysroot` flag simplifies an Android cross-compilation,
for example.
ARM JIT: fix software floating-point

I broke uses of LDRD and STRD when compacting the set of registers
used by the JIT. The LDRD and STRD instructions are given one
register explicitly, but they implicitly use the next regsister, too,
and the specified register must be even-numbered. Lining up a pair of
registers requires a little shuffling before and after the operation.

Also, the LDRDI and STRD encodings were broken, and the inlined
fl->fx conversion was not right.

Closes PR 14470
no getprotobyname on Android