[SYCL][ABI-break] Rename SYCL host-side math builtins

[steffenlarsen]

Due to a bug in GCC versions older than 11.1 SYCL math builtins may fail to compile when using GCC as the host compiler. To work around this issue, this commit renames all host-side builtin implementations used by the SYCL math builtins.

The bug:

#include <cmath>

namespace __host_std {
  template <typename R, typename T>
  inline R __invoke_sqrt(T a) {
    extern R sqrt(T);
    return sqrt(a);
  }
}

int main() {
  __host_std::__invoke_sqrt<double>(3.5);
  return 0;
}

With gcc 10.4:

<source>: In instantiation of 'R __host_std::__invoke_sqrt(T) [with R = double; T = double]':
<source>:12:42:   required from here
<source>:6:18: error: local external declaration 'R __host_std::sqrt(T) [with R = double; T = double]' [-fpermissive]
    6 |         extern R sqrt(T);
      |                  ^~~~
In file included from /usr/include/features.h:461,
                 from /opt/compiler-explorer/gcc-10.4.0/include/c++/10.4.0/x86_64-linux-gnu/bits/os_defines.h:39,
                 from /opt/compiler-explorer/gcc-10.4.0/include/c++/10.4.0/x86_64-linux-gnu/bits/c++config.h:522,
                 from /opt/compiler-explorer/gcc-10.4.0/include/c++/10.4.0/cmath:41,
                 from <source>:1:
/usr/include/x86_64-linux-gnu/bits/mathcalls.h:143:1: note: does not match previous declaration 'double sqrt(double)'
  143 | __MATHCALL (sqrt,, (_Mdouble_ __x));
      | ^~~~~~~~~~
Compiler returned: 1