C backend: generate GCC-compatible code

[andrewrk]

This issue can be closed when we add CI test coverage that compiles the behavior tests with -ofmt=c and then compiles & runs them with GCC.

Here's what happens currently, with GCC 11.3.0:

$ stage3/bin/zig test ../test/behavior.zig -I../test -ofmt=c -femit-bin=test.c
$ gcc -o test test.c
test.c: In function ‘behavior_atomics_testAtomicRmwFloat’:
test.c:19080:2: error: operand type ‘_Atomic zig_f32 *’ {aka ‘_Atomic float *’} is incompatible with argument 1 of ‘__atomic_fetch_add’
19080 |  zig_f32 const t10 = zig_atomicrmw_add((zig_atomic(zig_f32 ) *)&t0, (zig_f32 )zig_as_f32(0x1.4p2, zig_as_i32(0x40a00000)), memory_order_seq_cst);
      |  ^~~~~~~
test.c:19094:2: error: operand type ‘_Atomic zig_f32 *’ {aka ‘_Atomic float *’} is incompatible with argument 1 of ‘__atomic_fetch_sub’
19094 |  zig_f32 const t15 = zig_atomicrmw_sub((zig_atomic(zig_f32 ) *)&t0, (zig_f32 )zig_as_f32(0x1p1, zig_as_i32(0x40000000)), memory_order_seq_cst);
      |  ^~~~~~~
In file included from test.c:1:
test.c: In function ‘behavior_atomics_testAtomicRmwInt’:
zig.h:127:45: warning: implicit declaration of function ‘__atomic_fetch_max’; did you mean ‘__atomic_fetch_add’? [-Wimplicit-function-declaration]
  127 | #define  zig_atomicrmw_max(obj, arg, order) __atomic_fetch_max        (obj, arg, order)
      |                                             ^~~~~~~~~~~~~~~~~~
test.c:19263:21: note: in expansion of macro ‘zig_atomicrmw_max’
19263 |  zig_u8 const t41 = zig_atomicrmw_max((zig_u8  *)&t0, zig_as_u8(1), memory_order_seq_cst);
      |                     ^~~~~~~~~~~~~~~~~
zig.h:126:45: warning: implicit declaration of function ‘__atomic_fetch_min’; did you mean ‘__atomic_fetch_add’? [-Wimplicit-function-declaration]
  126 | #define  zig_atomicrmw_min(obj, arg, order) __atomic_fetch_min        (obj, arg, order)
      |                                             ^~~~~~~~~~~~~~~~~~
test.c:19277:21: note: in expansion of macro ‘zig_atomicrmw_min’
19277 |  zig_u8 const t46 = zig_atomicrmw_min((zig_u8  *)&t0, zig_as_u8(1), memory_order_seq_cst);
      |                     ^~~~~~~~~~~~~~~~~