[glsl-out] Polyfill frexp

[evahop]

Implementation derived from https://en.cppreference.com/w/cpp/numeric/math/frexp#Notes.

Produces the following in tests/out/glsl/math-functions.main.Fragment.glsl when targeting < 400 & 310 es.

45   │ __frexp_result_f32_ naga_frexp(float arg) {
46   │     int other = arg == float(0) ? int(0) : int(float(1) + log2(arg));
47   │     float fract = arg * exp2(float(-other));
48   │     return __frexp_result_f32_(fract, other);
49   │ }
50   │
51   │ __frexp_result_vec4_f32_ naga_frexp(vec4 arg) {
52   │     ivec4 other = arg == vec4(0) ? ivec4(0) : ivec4(vec4(1) + log2(arg));
53   │     vec4 fract = arg * exp2(vec4(-other));
54   │     return __frexp_result_vec4_f32_(fract, other);
55   │ }

[teoxoy]

LGTM