fixes for RISC-V ABI Implementation #1567 (#2937)

* fixes for RISC-V ABI Implementation #1567

Fixed RISC-V floating-point ABI by correcting the target triple to
`riscv64-unknown-linux-gnu`, adding the `target-abi` module flag, and
ensuring ABI-required CPU features are enabled.

I tested this with:
```bash
build/c3c compile-only --target linux-riscv64 rv_hello.c3
readelf -h obj/linux-riscv64/rv_hello.o | grep Flags
# Output: Flags: 0x5, RVC, double-float ABI
```

```bash
# and qemu because I don't have a riscv machine :/
qemu-riscv64-static -L /usr/riscv64-linux-gnu ./rv_hello
```

---

@lerno I purposedly left these two failing tests to clearly see the
difference.

`test/test_suite/abi/riscv64-lp64-lp64f-abi-1.c3t`
`test/test_suite/abi/riscv64-lp64-abi.c3t`

* improve Linux cross-compilation, specifically for RISC-V

- Implement automatic sysroot and CRT object discovery for RISC-V.
- Fix dynamic linker paths and emulation flags for LLD.
- Link against libgcc to resolve required arithmetic symbols.

* Update tests.

* fix linker CRT detection by centralizing host arch check

use target_host_arch() in linker.c
move target_host_arch() from hostinfo.c to target.c

* missing debug info

---------

Co-authored-by: Christoffer Lerno <christoffer@aegik.com>

test/test_suite/abi/riscv64-lp64-lp64f-abi-1.c3t

@@ -24,7 +24,7 @@ fn Large f_scalar_stack_2(double a, int128 b, float128 c, Char32V d, char e, ich
 
 /* #expect: test.ll
 
-define signext i32 @test.f_scalar_stack_1(i32 signext %0, i128 %1, double %2, fp128 %3, ptr align 32 %4, i8 zeroext %5, i8 %6, i8 %7) #0 {
+define signext i32 @test.f_scalar_stack_1(i32 signext %0, i128 %1, double %2, fp128 %3, ptr align 32 %4, i8 zeroext %5, i8 signext %6, i8 %7) #0 {
 entry:
   %sext = sext i8 %6 to i32
   %zext = zext i8 %7 to i32
@@ -32,8 +32,7 @@ entry:
   ret i32 %add
 }
 
-; Function Attrs:
-define void @test.f_scalar_stack_2(ptr noalias sret(%Large) align 8 %0, double %1, i128 %2, fp128 %3, ptr align 32 %4, i8 zeroext %5, i8 %6, i8 %7) #0 {
+define void @test.f_scalar_stack_2(ptr noalias sret(%Large) align 8 %0, double %1, i128 %2, fp128 %3, ptr align 32 %4, i8 zeroext %5, i8 signext %6, i8 %7) #0 {
 entry:
   %literal = alloca %Large, align 8
   %fpsi = fptosi double %1 to i64