CL 68490 reworked the way the panicmem function is exposed to the
C mach expection catcher. However, //go:cgo_export_static isn't enough:
the underlying assembly functions must not start with the middle dot.

Without the middle dot, the panicmem function's exported name is
not prefixed with its package; rename it to xx_cgo_panicmem to decrease
the chance of a symbol name clash.

Finally, mark the overridden C symbol weak to avoid duplicate symbol
errors from the host linker.

For the ios builders.

Change-Id: Ib87789fecec9314e398cf1bd8c04ba0b3a6642af
Reviewed-on: https://go-review.googlesource.com/69113
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>