build: fix x86_64 asm detection for some compilers

[theuni]

I'm not familiar enough with this asm syntax, there's probably a much simpler solution here, just switching to a test that doesn't need escaping.

I Noticed this on OSX with clang, though it likely happens elsewhere as well.
The result is disabled x86_64 asm.

Due to missing escaping, this $0 was interpreted as the function name
SECP_64BIT_ASM_CHECK, causing the compile-check to be broken on some compilers.

The actual check looked like this:

int main()
{
  uint64_t a = 11, tmp;
  __asm__ __volatile__("movq SECP_64BIT_ASM_CHECKx100000000,%1; mulq %%rsi" : "+a"(a) : "S"(tmp) : "cc", "%rdx");
  return 0;
}

It seems even more odd that it compiled anywhere.

[theuni]

I just double-checked this to see if i needed to push for urgent merge here for Bitcoin 0.12rc1.

For reference, this does not affect the clang version currently used for Bitcoin OSX release builds. So no rush.