selftests/powerpc: Allow building without static libc

Currently exec-target.c is linked statically with libc, which on Fedora
at least requires installing an additional package (glibc-static).

If that package is not installed the build fails with:

    CC       exec_target
  /usr/bin/ld: cannot find -lc: No such file or directory
  collect2: error: ld returned 1 exit status

All exec_target.c does is call sys_exit, which can be done easily enough
using inline assembly, and removes the requirement for a static libc to
be installed.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240812094152.418586-1-maddy@linux.ibm.com

Author: maddy-kerneldev

tools/testing/selftests/powerpc/benchmarks/Makefile

@@ -18,4 +18,4 @@ $(OUTPUT)/context_switch: LDLIBS += -lpthread
 
 $(OUTPUT)/fork: LDLIBS += -lpthread
 
-$(OUTPUT)/exec_target: CFLAGS += -static -nostartfiles
+$(OUTPUT)/exec_target: CFLAGS += -nostartfiles

tools/testing/selftests/powerpc/benchmarks/exec_target.c

@@ -7,10 +7,22 @@
  */
 
 #define _GNU_SOURCE
-#include <unistd.h>
 #include <sys/syscall.h>
 
 void _start(void)
 {
-	syscall(SYS_exit, 0);
+	asm volatile (
+		"li %%r0, %[sys_exit];"
+		"li %%r3, 0;"
+		"sc;"
+		:
+		: [sys_exit] "i" (SYS_exit)
+		/*
+		 * "sc" will clobber r0, r3-r13, cr0, ctr, xer and memory.
+		 * Even though sys_exit never returns, handle clobber
+		 * registers.
+		 */
+		: "r0", "r3", "r4", "r5", "r6", "r7", "r8", "r9", "r10",
+		  "r11", "r12", "r13", "cr0", "ctr", "xer", "memory"
+	);
 }