syscall: call getfsstat via libc on openbsd

On openbsd, call getfsstat directly via libc, instead of calling it
via syscall.Syscall.

Updates golang/go#63900

Change-Id: I99a3555b3207754b107dbabf63440569fca93eea
Reviewed-on: https://go-review.googlesource.com/c/sys/+/538976
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Josh Rickmar <jrick@zettaport.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
diff --git a/unix/zsyscall_openbsd_riscv64.s b/unix/zsyscall_openbsd_riscv64.s
index 27130c6..87a79c7 100644
--- a/unix/zsyscall_openbsd_riscv64.s
+++ b/unix/zsyscall_openbsd_riscv64.s
@@ -668,6 +668,11 @@
 GLOBL	·libc_munmap_trampoline_addr(SB), RODATA, $8
 DATA	·libc_munmap_trampoline_addr(SB)/8, $libc_munmap_trampoline<>(SB)
 
+TEXT libc_getfsstat_trampoline<>(SB),NOSPLIT,$0-0
+	JMP	libc_getfsstat(SB)
+GLOBL	·libc_getfsstat_trampoline_addr(SB), RODATA, $8
+DATA	·libc_getfsstat_trampoline_addr(SB)/8, $libc_getfsstat_trampoline<>(SB)
+
 TEXT libc_utimensat_trampoline<>(SB),NOSPLIT,$0-0
 	JMP	libc_utimensat(SB)
 GLOBL	·libc_utimensat_trampoline_addr(SB), RODATA, $8