Fix building on OpenBSD

[realchonk]

I was able to build and use mrustc on OpenBSD, but with a few changes.
To be able to use mrustc, I had to set CC=egcc (gcc-11.2.0),
because compiler_builtins would emit wrong assembly for clang.

Please test my changes, so they don't affect other systems.

compiler_builtins failure with clang:

--- BUILDING compiler_builtins v0.1.101 (15.8% 1r,0w,15b,3c/19t)
> tools/bin/mrustc rustc-1.74.0-src/vendor/compiler_builtins/src/lib.rs -o output-1.74.0/libcompiler_builtins-0_1_101_H10c.rlib -C emit-depfile=output-1.74.0/libcompiler_builtins-0_1_101_H10c.rlib.d --cfg debug_assertions -O -L output-1.74.0 --cfg feature="compiler-builtins" --cfg feature="rustc-dep-of-std" --cfg feature="core" --cfg feature="dep:core" --crate-name compiler_builtins --crate-type rlib --crate-tag 0_1_101_H10c --cfg feature="unstable" --edition 2018 --extern core=output-1.74.0/librustc_std_workspace_core-1_99_0.rlib > output-1.74.0/libcompiler_builtins-0_1_101_H10c.rlib_dbg.txt
 (15.8% 1r,0w,15b,3c/19t): compiler_builtins v0.1.101
output-1.74.0/libcompiler_builtins-0_1_101_H10c.rlib.c:14252:83: error: invalid input constraint '0' in asm
        __asm__ __volatile__("div %2;\n " : "+r" (asm_rax), "+r" (asm_rdx) : "r" (arg1), "0" (asm_rax), "1" (asm_rdx):);
                                                                                         ^
output-1.74.0/libcompiler_builtins-0_1_101_H10c.rlib.c:14349:84: error: invalid input constraint '0' in asm
        __asm__ __volatile__("div %2;\n " : "+r" (asm_rax), "+r" (asm_rdx) : "r" (var38), "0" (asm_rax), "1" (asm_rdx):);
                                                                                          ^
output-1.74.0/libcompiler_builtins-0_1_101_H10c.rlib.c:14399:83: error: invalid input constraint '0' in asm
        __asm__ __volatile__("div %2;\n " : "+r" (asm_rax), "+r" (asm_rdx) : "r" (var3), "0" (asm_rax), "1" (asm_rdx):);
                                                                                         ^
output-1.74.0/libcompiler_builtins-0_1_101_H10c.rlib.c:14424:83: error: invalid input constraint '0' in asm
        __asm__ __volatile__("div %2;\n " : "+r" (asm_rax), "+r" (asm_rdx) : "r" (var3), "0" (asm_rax), "1" (asm_rdx):);
                                                                                         ^
output-1.74.0/libcompiler_builtins-0_1_101_H10c.rlib.c:16076:89: error: invalid input constraint '0' in asm
        __asm__ __volatile__("rep movsb;\n " : "+r" (asm_rdi), "+r" (asm_rsi) : "r" (asm_ecx), "0" (asm_rdi), "1" (asm_rsi):);
                                                                                               ^
output-1.74.0/libcompiler_builtins-0_1_101_H10c.rlib.c:16087:89: error: invalid input constraint '0' in asm
        __asm__ __volatile__("rep movsq;\n " : "+r" (asm_rdi), "+r" (asm_rsi) : "r" (asm_rcx), "0" (asm_rdi), "1" (asm_rsi):);
                                                                                               ^
output-1.74.0/libcompiler_builtins-0_1_101_H10c.rlib.c:16145:73: error: invalid input constraint '0' in asm
        __asm__ __volatile__("rep stosb;\n " : "+r" (asm_rdi) : "r" (asm_ecx), "0" (asm_rdi), "r" (asm_rax):);
                                                                               ^
output-1.74.0/libcompiler_builtins-0_1_101_H10c.rlib.c:16154:73: error: invalid input constraint '0' in asm
        __asm__ __volatile__("rep stosq;\n " : "+r" (asm_rdi) : "r" (asm_rcx), "0" (asm_rdi), "r" (asm_rax):);
                                                                               ^
8 errors generated.
C Compiler failed to execute - error code 256
Process exited with non-zero exit status 1
FAILING COMMAND: tools/bin/mrustc rustc-1.74.0-src/vendor/compiler_builtins/src/lib.rs -o output-1.74.0/libcompiler_builtins-0_1_101_H10c.rlib -C emit-depfile=output-1.74.0/libcompiler_builtins-0_1_101_H10c.rlib.d --cfg debug_assertions
-O -L output-1.74.0 --cfg feature="compiler-builtins" --cfg feature="rustc-dep-of-std" --cfg feature="core" --cfg feature="dep:core" --crate-name compiler_builtins --crate-type rlib --crate-tag 0_1_101_H10c --cfg feature="unstable" --edition 2018 --extern core=output-1.74.0/librustc_std_workspace_core-1_99_0.rlib