Enable SIMD register calling convention with gmx_simdcall

Cmake now checks if the compiler supports __vectorcall or
__regcall calling convention modifiers, and sets gmx_simdcall
to one of these if supported, otherwise a blank string.
This should enable 32-bit MSVC to accept our SIMD routines
(starting from MSVC 2013), and with ICC it can at least in
theory improve performance slightly by using more registers
for argument passing in 64-bit mode too. Presently this is
only useful on x86, but the infrastructure will work if we
find similar calling conventions on other architectures.

Fixes #1541.

Change-Id: I7026fb4e1fb6b88c8aa18b060a631cbb80231cd4