builtin: guard linux_bare amd64 syscalls with $if amd64

The amd64 Linux syscall layer in linux_bare was gated on `$if !i386`, which
the C backend emits as `#if !defined(__V_i386)` -- true for arm/arm64/riscv/etc.
Those targets then tried to compile `asm amd64 { syscall ... }`, failing with
"impossible constraint in 'asm'" when cross-compiling -freestanding output
(e.g. for bare-metal Cortex-M with arm-none-eabi-gcc).

Gate it on `$if amd64` instead: the block uses the amd64 syscall ABI and the
`syscall` instruction, i386 has its own linux_syscalls.i386.v, and this matches
how arch-specific asm is guarded elsewhere in vlib. amd64 output is unchanged.

Author: MartenH

vlib/builtin/linux_bare/linux_syscalls.v

@@ -223,7 +223,7 @@ enum WiSiCode {
 	cld_continued = 6 // stopped child has continued
 }
 
-$if !i386 {
+$if amd64 {
 	fn split_int_errno(rc_in u64) (i64, Errno) {
 		rc := i64(rc_in)
 		if rc < 0 {